-
Notifications
You must be signed in to change notification settings - Fork 2
Expand file tree
/
Copy pathreact_doc.html
More file actions
1597 lines (1561 loc) · 175 KB
/
react_doc.html
File metadata and controls
1597 lines (1561 loc) · 175 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<meta http-equiv="Content-Style-Type" content="text/css" />
<meta name="generator" content="pandoc" />
<title></title>
<style type="text/css">code{white-space: pre;}</style>
<style type="text/css">
div.sourceCode { overflow-x: auto; }
table.sourceCode, tr.sourceCode, td.lineNumbers, td.sourceCode {
margin: 0; padding: 0; vertical-align: baseline; border: none; }
table.sourceCode { width: 100%; line-height: 100%; }
td.lineNumbers { text-align: right; padding-right: 4px; padding-left: 4px; color: #aaaaaa; border-right: 1px solid #aaaaaa; }
td.sourceCode { padding-left: 5px; }
code > span.kw { color: #007020; font-weight: bold; } /* Keyword */
code > span.dt { color: #902000; } /* DataType */
code > span.dv { color: #40a070; } /* DecVal */
code > span.bn { color: #40a070; } /* BaseN */
code > span.fl { color: #40a070; } /* Float */
code > span.ch { color: #4070a0; } /* Char */
code > span.st { color: #4070a0; } /* String */
code > span.co { color: #60a0b0; font-style: italic; } /* Comment */
code > span.ot { color: #007020; } /* Other */
code > span.al { color: #ff0000; font-weight: bold; } /* Alert */
code > span.fu { color: #06287e; } /* Function */
code > span.er { color: #ff0000; font-weight: bold; } /* Error */
code > span.wa { color: #60a0b0; font-weight: bold; font-style: italic; } /* Warning */
code > span.cn { color: #880000; } /* Constant */
code > span.sc { color: #4070a0; } /* SpecialChar */
code > span.vs { color: #4070a0; } /* VerbatimString */
code > span.ss { color: #bb6688; } /* SpecialString */
code > span.im { } /* Import */
code > span.va { color: #19177c; } /* Variable */
code > span.cf { color: #007020; font-weight: bold; } /* ControlFlow */
code > span.op { color: #666666; } /* Operator */
code > span.bu { } /* BuiltIn */
code > span.ex { } /* Extension */
code > span.pp { color: #bc7a00; } /* Preprocessor */
code > span.at { color: #7d9029; } /* Attribute */
code > span.do { color: #ba2121; font-style: italic; } /* Documentation */
code > span.an { color: #60a0b0; font-weight: bold; font-style: italic; } /* Annotation */
code > span.cv { color: #60a0b0; font-weight: bold; font-style: italic; } /* CommentVar */
code > span.in { color: #60a0b0; font-weight: bold; font-style: italic; } /* Information */
</style>
</head>
<body>
<p>This project was bootstrapped with <a href="https://github.com/facebookincubator/create-react-app">Create React App</a>.</p>
<p>Below you will find some information on how to perform common tasks.<br> You can find the most recent version of this guide <a href="https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md">here</a>.</p>
<h2 id="table-of-contents">Table of Contents</h2>
<ul>
<li><a href="#updating-to-new-releases">Updating to New Releases</a></li>
<li><a href="#sending-feedback">Sending Feedback</a></li>
<li><a href="#folder-structure">Folder Structure</a></li>
<li><a href="#available-scripts">Available Scripts</a></li>
<li><a href="#npm-start">npm start</a></li>
<li><a href="#npm-test">npm test</a></li>
<li><a href="#npm-run-build">npm run build</a></li>
<li><a href="#npm-run-eject">npm run eject</a></li>
<li><a href="#supported-browsers">Supported Browsers</a></li>
<li><a href="#supported-language-features-and-polyfills">Supported Language Features and Polyfills</a></li>
<li><a href="#syntax-highlighting-in-the-editor">Syntax Highlighting in the Editor</a></li>
<li><a href="#displaying-lint-output-in-the-editor">Displaying Lint Output in the Editor</a></li>
<li><a href="#debugging-in-the-editor">Debugging in the Editor</a></li>
<li><a href="#formatting-code-automatically">Formatting Code Automatically</a></li>
<li><a href="#changing-the-page-title">Changing the Page <code><title></code></a></li>
<li><a href="#installing-a-dependency">Installing a Dependency</a></li>
<li><a href="#importing-a-component">Importing a Component</a></li>
<li><a href="#code-splitting">Code Splitting</a></li>
<li><a href="#adding-a-stylesheet">Adding a Stylesheet</a></li>
<li><a href="#post-processing-css">Post-Processing CSS</a></li>
<li><a href="#adding-a-css-preprocessor-sass-less-etc">Adding a CSS Preprocessor (Sass, Less etc.)</a></li>
<li><a href="#adding-images-fonts-and-files">Adding Images, Fonts, and Files</a></li>
<li><a href="#using-the-public-folder">Using the <code>public</code> Folder</a></li>
<li><a href="#changing-the-html">Changing the HTML</a></li>
<li><a href="#adding-assets-outside-of-the-module-system">Adding Assets Outside of the Module System</a></li>
<li><a href="#when-to-use-the-public-folder">When to Use the <code>public</code> Folder</a></li>
<li><a href="#using-global-variables">Using Global Variables</a></li>
<li><a href="#adding-bootstrap">Adding Bootstrap</a></li>
<li><a href="#using-a-custom-theme">Using a Custom Theme</a></li>
<li><a href="#adding-flow">Adding Flow</a></li>
<li><a href="#adding-a-router">Adding a Router</a></li>
<li><a href="#adding-custom-environment-variables">Adding Custom Environment Variables</a></li>
<li><a href="#referencing-environment-variables-in-the-html">Referencing Environment Variables in the HTML</a></li>
<li><a href="#adding-temporary-environment-variables-in-your-shell">Adding Temporary Environment Variables In Your Shell</a></li>
<li><a href="#adding-development-environment-variables-in-env">Adding Development Environment Variables In <code>.env</code></a></li>
<li><a href="#can-i-use-decorators">Can I Use Decorators?</a></li>
<li><a href="#fetching-data-with-ajax-requests">Fetching Data with AJAX Requests</a></li>
<li><a href="#integrating-with-an-api-backend">Integrating with an API Backend</a></li>
<li><a href="#node">Node</a></li>
<li><a href="#ruby-on-rails">Ruby on Rails</a></li>
<li><a href="#proxying-api-requests-in-development">Proxying API Requests in Development</a></li>
<li><a href="#invalid-host-header-errors-after-configuring-proxy">"Invalid Host Header" Errors After Configuring Proxy</a></li>
<li><a href="#configuring-the-proxy-manually">Configuring the Proxy Manually</a></li>
<li><a href="#configuring-a-websocket-proxy">Configuring a WebSocket Proxy</a></li>
<li><a href="#using-https-in-development">Using HTTPS in Development</a></li>
<li><a href="#generating-dynamic-meta-tags-on-the-server">Generating Dynamic <code><meta></code> Tags on the Server</a></li>
<li><a href="#pre-rendering-into-static-html-files">Pre-Rendering into Static HTML Files</a></li>
<li><a href="#injecting-data-from-the-server-into-the-page">Injecting Data from the Server into the Page</a></li>
<li><a href="#running-tests">Running Tests</a></li>
<li><a href="#filename-conventions">Filename Conventions</a></li>
<li><a href="#command-line-interface">Command Line Interface</a></li>
<li><a href="#version-control-integration">Version Control Integration</a></li>
<li><a href="#writing-tests">Writing Tests</a></li>
<li><a href="#testing-components">Testing Components</a></li>
<li><a href="#using-third-party-assertion-libraries">Using Third Party Assertion Libraries</a></li>
<li><a href="#initializing-test-environment">Initializing Test Environment</a></li>
<li><a href="#focusing-and-excluding-tests">Focusing and Excluding Tests</a></li>
<li><a href="#coverage-reporting">Coverage Reporting</a></li>
<li><a href="#continuous-integration">Continuous Integration</a></li>
<li><a href="#disabling-jsdom">Disabling jsdom</a></li>
<li><a href="#snapshot-testing">Snapshot Testing</a></li>
<li><a href="#editor-integration">Editor Integration</a></li>
<li><a href="#debugging-tests">Debugging Tests</a></li>
<li><a href="#debugging-tests-in-chrome">Debugging Tests in Chrome</a></li>
<li><a href="#debugging-tests-in-visual-studio-code">Debugging Tests in Visual Studio Code</a></li>
<li><a href="#developing-components-in-isolation">Developing Components in Isolation</a></li>
<li><a href="#getting-started-with-storybook">Getting Started with Storybook</a></li>
<li><a href="#getting-started-with-styleguidist">Getting Started with Styleguidist</a></li>
<li><a href="#publishing-components-to-npm">Publishing Components to npm</a></li>
<li><a href="#making-a-progressive-web-app">Making a Progressive Web App</a></li>
<li><a href="#opting-out-of-caching">Opting Out of Caching</a></li>
<li><a href="#offline-first-considerations">Offline-First Considerations</a></li>
<li><a href="#progressive-web-app-metadata">Progressive Web App Metadata</a></li>
<li><a href="#analyzing-the-bundle-size">Analyzing the Bundle Size</a></li>
<li><a href="#deployment">Deployment</a></li>
<li><a href="#static-server">Static Server</a></li>
<li><a href="#other-solutions">Other Solutions</a></li>
<li><a href="#serving-apps-with-client-side-routing">Serving Apps with Client-Side Routing</a></li>
<li><a href="#building-for-relative-paths">Building for Relative Paths</a></li>
<li><a href="#azure">Azure</a></li>
<li><a href="#firebase">Firebase</a></li>
<li><a href="#github-pages">GitHub Pages</a></li>
<li><a href="#heroku">Heroku</a></li>
<li><a href="#netlify">Netlify</a></li>
<li><a href="#now">Now</a></li>
<li><a href="#s3-and-cloudfront">S3 and CloudFront</a></li>
<li><a href="#surge">Surge</a></li>
<li><a href="#advanced-configuration">Advanced Configuration</a></li>
<li><a href="#troubleshooting">Troubleshooting</a></li>
<li><a href="#npm-start-doesnt-detect-changes"><code>npm start</code> doesn’t detect changes</a></li>
<li><a href="#npm-test-hangs-on-macos-sierra"><code>npm test</code> hangs on macOS Sierra</a></li>
<li><a href="#npm-run-build-exits-too-early"><code>npm run build</code> exits too early</a></li>
<li><a href="#npm-run-build-fails-on-heroku"><code>npm run build</code> fails on Heroku</a></li>
<li><a href="#npm-run-build-fails-to-minify"><code>npm run build</code> fails to minify</a></li>
<li><a href="#momentjs-locales-are-missing">Moment.js locales are missing</a></li>
<li><a href="#alternatives-to-ejecting">Alternatives to Ejecting</a></li>
<li><a href="#something-missing">Something Missing?</a></li>
</ul>
<h2 id="updating-to-new-releases">Updating to New Releases</h2>
<p>Create React App is divided into two packages:</p>
<ul>
<li><code>create-react-app</code> is a global command-line utility that you use to create new projects.</li>
<li><code>react-scripts</code> is a development dependency in the generated projects (including this one).</li>
</ul>
<p>You almost never need to update <code>create-react-app</code> itself: it delegates all the setup to <code>react-scripts</code>.</p>
<p>When you run <code>create-react-app</code>, it always creates the project with the latest version of <code>react-scripts</code> so you’ll get all the new features and improvements in newly created apps automatically.</p>
<p>To update an existing project to a new version of <code>react-scripts</code>, <a href="https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md">open the changelog</a>, find the version you’re currently on (check <code>package.json</code> in this folder if you’re not sure), and apply the migration instructions for the newer versions.</p>
<p>In most cases bumping the <code>react-scripts</code> version in <code>package.json</code> and running <code>npm install</code> in this folder should be enough, but it’s good to consult the <a href="https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md">changelog</a> for potential breaking changes.</p>
<p>We commit to keeping the breaking changes minimal so you can upgrade <code>react-scripts</code> painlessly.</p>
<h2 id="sending-feedback">Sending Feedback</h2>
<p>We are always open to <a href="https://github.com/facebookincubator/create-react-app/issues">your feedback</a>.</p>
<h2 id="folder-structure">Folder Structure</h2>
<p>After creation, your project should look like this:</p>
<pre><code>my-app/
README.md
node_modules/
package.json
public/
index.html
favicon.ico
src/
App.css
App.js
App.test.js
index.css
index.js
logo.svg</code></pre>
<p>For the project to build, <strong>these files must exist with exact filenames</strong>:</p>
<ul>
<li><code>public/index.html</code> is the page template;</li>
<li><code>src/index.js</code> is the JavaScript entry point.</li>
</ul>
<p>You can delete or rename the other files.</p>
<p>You may create subdirectories inside <code>src</code>. For faster rebuilds, only files inside <code>src</code> are processed by Webpack.<br> You need to <strong>put any JS and CSS files inside <code>src</code></strong>, otherwise Webpack won’t see them.</p>
<p>Only files inside <code>public</code> can be used from <code>public/index.html</code>.<br> Read instructions below for using assets from JavaScript and HTML.</p>
<p>You can, however, create more top-level directories.<br> They will not be included in the production build so you can use them for things like documentation.</p>
<h2 id="available-scripts">Available Scripts</h2>
<p>In the project directory, you can run:</p>
<h3 id="npm-start"><code>npm start</code></h3>
<p>Runs the app in the development mode.<br> Open <a href="http://localhost:3000" class="uri">http://localhost:3000</a> to view it in the browser.</p>
<p>The page will reload if you make edits.<br> You will also see any lint errors in the console.</p>
<h3 id="npm-test"><code>npm test</code></h3>
<p>Launches the test runner in the interactive watch mode.<br> See the section about <a href="#running-tests">running tests</a> for more information.</p>
<h3 id="npm-run-build"><code>npm run build</code></h3>
<p>Builds the app for production to the <code>build</code> folder.<br> It correctly bundles React in production mode and optimizes the build for the best performance.</p>
<p>The build is minified and the filenames include the hashes.<br> Your app is ready to be deployed!</p>
<p>See the section about <a href="#deployment">deployment</a> for more information.</p>
<h3 id="npm-run-eject"><code>npm run eject</code></h3>
<p><strong>Note: this is a one-way operation. Once you <code>eject</code>, you can’t go back!</strong></p>
<p>If you aren’t satisfied with the build tool and configuration choices, you can <code>eject</code> at any time. This command will remove the single build dependency from your project.</p>
<p>Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except <code>eject</code> will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.</p>
<p>You don’t have to ever use <code>eject</code>. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.</p>
<h2 id="supported-browsers">Supported Browsers</h2>
<p>By default, the generated project uses the latest version of React.</p>
<p>You can refer <a href="https://reactjs.org/docs/react-dom.html#browser-support">to the React documentation</a> for more information about supported browsers.</p>
<h2 id="supported-language-features-and-polyfills">Supported Language Features and Polyfills</h2>
<p>This project supports a superset of the latest JavaScript standard.<br> In addition to <a href="https://github.com/lukehoban/es6features">ES6</a> syntax features, it also supports:</p>
<ul>
<li><a href="https://github.com/rwaldron/exponentiation-operator">Exponentiation Operator</a> (ES2016).</li>
<li><a href="https://github.com/tc39/ecmascript-asyncawait">Async/await</a> (ES2017).</li>
<li><a href="https://github.com/sebmarkbage/ecmascript-rest-spread">Object Rest/Spread Properties</a> (stage 3 proposal).</li>
<li><a href="https://github.com/tc39/proposal-dynamic-import">Dynamic import()</a> (stage 3 proposal)</li>
<li><a href="https://github.com/tc39/proposal-class-public-fields">Class Fields and Static Properties</a> (part of stage 3 proposal).</li>
<li><a href="https://facebook.github.io/react/docs/introducing-jsx.html">JSX</a> and <a href="https://flowtype.org/">Flow</a> syntax.</li>
</ul>
<p>Learn more about <a href="https://babeljs.io/docs/plugins/#presets-stage-x-experimental-presets-">different proposal stages</a>.</p>
<p>While we recommend using experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide <a href="https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb">codemods</a> if any of these proposals change in the future.</p>
<p>Note that <strong>the project only includes a few ES6 <a href="https://en.wikipedia.org/wiki/Polyfill">polyfills</a></strong>:</p>
<ul>
<li><a href="https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign"><code>Object.assign()</code></a> via <a href="https://github.com/sindresorhus/object-assign"><code>object-assign</code></a>.</li>
<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise"><code>Promise</code></a> via <a href="https://github.com/then/promise"><code>promise</code></a>.</li>
<li><a href="https://developer.mozilla.org/en/docs/Web/API/Fetch_API"><code>fetch()</code></a> via <a href="https://github.com/github/fetch"><code>whatwg-fetch</code></a>.</li>
</ul>
<p>If you use any other ES6+ features that need <strong>runtime support</strong> (such as <code>Array.from()</code> or <code>Symbol</code>), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.</p>
<p>Also note that using some newer syntax features like <code>for...of</code> or <code>[...nonArrayValue]</code> causes Babel to emit code that depends on ES6 runtime features and might not work without a polyfill. When in doubt, use <a href="https://babeljs.io/repl/">Babel REPL</a> to see what any specific syntax compiles down to.</p>
<h2 id="syntax-highlighting-in-the-editor">Syntax Highlighting in the Editor</h2>
<p>To configure the syntax highlighting in your favorite text editor, head to the <a href="https://babeljs.io/docs/editors">relevant Babel documentation page</a> and follow the instructions. Some of the most popular editors are covered.</p>
<h2 id="displaying-lint-output-in-the-editor">Displaying Lint Output in the Editor</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.2.0</code> and higher.<br> It also only works with npm 3 or higher.</p>
</blockquote>
<p>Some editors, including Sublime Text, Atom, and Visual Studio Code, provide plugins for ESLint.</p>
<p>They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.</p>
<p>You would need to install an ESLint plugin for your editor first. Then, add a file called <code>.eslintrc</code> to the project root:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="op">{</span>
<span class="st">"extends"</span><span class="op">:</span> <span class="st">"react-app"</span>
<span class="op">}</span></code></pre></div>
<p>Now your editor should report the linting warnings.</p>
<p>Note that even if you edit your <code>.eslintrc</code> file further, these changes will <strong>only affect the editor integration</strong>. They won’t affect the terminal and in-browser lint output. This is because Create React App intentionally provides a minimal set of rules that find common mistakes.</p>
<p>If you want to enforce a coding style for your project, consider using <a href="https://github.com/jlongster/prettier">Prettier</a> instead of ESLint style rules.</p>
<h2 id="debugging-in-the-editor">Debugging in the Editor</h2>
<p><strong>This feature is currently only supported by <a href="https://code.visualstudio.com">Visual Studio Code</a> and <a href="https://www.jetbrains.com/webstorm/">WebStorm</a>.</strong></p>
<p>Visual Studio Code and WebStorm support debugging out of the box with Create React App. This enables you as a developer to write and debug your React code without leaving the editor, and most importantly it enables you to have a continuous development workflow, where context switching is minimal, as you don’t have to switch between tools.</p>
<h3 id="visual-studio-code">Visual Studio Code</h3>
<p>You would need to have the latest version of <a href="https://code.visualstudio.com">VS Code</a> and VS Code <a href="https://marketplace.visualstudio.com/items?itemName=msjsdiag.debugger-for-chrome">Chrome Debugger Extension</a> installed.</p>
<p>Then add the block below to your <code>launch.json</code> file and put it inside the <code>.vscode</code> folder in your app’s root directory.</p>
<div class="sourceCode"><pre class="sourceCode json"><code class="sourceCode json"><span class="fu">{</span>
<span class="dt">"version"</span><span class="fu">:</span> <span class="st">"0.2.0"</span><span class="fu">,</span>
<span class="dt">"configurations"</span><span class="fu">:</span> <span class="ot">[</span><span class="fu">{</span>
<span class="dt">"name"</span><span class="fu">:</span> <span class="st">"Chrome"</span><span class="fu">,</span>
<span class="dt">"type"</span><span class="fu">:</span> <span class="st">"chrome"</span><span class="fu">,</span>
<span class="dt">"request"</span><span class="fu">:</span> <span class="st">"launch"</span><span class="fu">,</span>
<span class="dt">"url"</span><span class="fu">:</span> <span class="st">"http://localhost:3000"</span><span class="fu">,</span>
<span class="dt">"webRoot"</span><span class="fu">:</span> <span class="st">"${workspaceRoot}/src"</span><span class="fu">,</span>
<span class="dt">"sourceMapPathOverrides"</span><span class="fu">:</span> <span class="fu">{</span>
<span class="dt">"webpack:///src/*"</span><span class="fu">:</span> <span class="st">"${webRoot}/*"</span>
<span class="fu">}</span>
<span class="fu">}</span><span class="ot">]</span>
<span class="fu">}</span></code></pre></div>
<blockquote>
<p>Note: the URL may be different if you've made adjustments via the <a href="#advanced-configuration">HOST or PORT environment variables</a>.</p>
</blockquote>
<p>Start your app by running <code>npm start</code>, and start debugging in VS Code by pressing <code>F5</code> or by clicking the green debug icon. You can now write code, set breakpoints, make changes to the code, and debug your newly modified code—all from your editor.</p>
<p>Having problems with VS Code Debugging? Please see their <a href="https://github.com/Microsoft/vscode-chrome-debug/blob/master/README.md#troubleshooting">troubleshooting guide</a>.</p>
<h3 id="webstorm">WebStorm</h3>
<p>You would need to have <a href="https://www.jetbrains.com/webstorm/">WebStorm</a> and <a href="https://chrome.google.com/webstore/detail/jetbrains-ide-support/hmhgeddbohgjknpmjagkdomcpobmllji">JetBrains IDE Support</a> Chrome extension installed.</p>
<p>In the WebStorm menu <code>Run</code> select <code>Edit Configurations...</code>. Then click <code>+</code> and select <code>JavaScript Debug</code>. Paste <code>http://localhost:3000</code> into the URL field and save the configuration.</p>
<blockquote>
<p>Note: the URL may be different if you've made adjustments via the <a href="#advanced-configuration">HOST or PORT environment variables</a>.</p>
</blockquote>
<p>Start your app by running <code>npm start</code>, then press <code>^D</code> on macOS or <code>F9</code> on Windows and Linux or click the green debug icon to start debugging in WebStorm.</p>
<p>The same way you can debug your application in IntelliJ IDEA Ultimate, PhpStorm, PyCharm Pro, and RubyMine.</p>
<h2 id="formatting-code-automatically">Formatting Code Automatically</h2>
<p>Prettier is an opinionated code formatter with support for JavaScript, CSS and JSON. With Prettier you can format the code you write automatically to ensure a code style within your project. See the <a href="https://github.com/prettier/prettier">Prettier's GitHub page</a> for more information, and look at this <a href="https://prettier.github.io/prettier/">page to see it in action</a>.</p>
<p>To format our code whenever we make a commit in git, we need to install the following dependencies:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save husky lint-staged prettier</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add husky lint-staged prettier</code></pre></div>
<ul>
<li><code>husky</code> makes it easy to use githooks as if they are npm scripts.</li>
<li><code>lint-staged</code> allows us to run scripts on staged files in git. See this <a href="https://medium.com/@okonetchnikov/make-linting-great-again-f3890e1ad6b8">blog post about lint-staged to learn more about it</a>.</li>
<li><code>prettier</code> is the JavaScript formatter we will run before commits.</li>
</ul>
<p>Now we can make sure every file is formatted correctly by adding a few lines to the <code>package.json</code> in the project root.</p>
<p>Add the following line to <code>scripts</code> section:</p>
<div class="sourceCode"><pre class="sourceCode diff"><code class="sourceCode diff"> "scripts": {
<span class="va">+ "precommit": "lint-staged",</span>
"start": "react-scripts start",
"build": "react-scripts build",</code></pre></div>
<p>Next we add a 'lint-staged' field to the <code>package.json</code>, for example:</p>
<div class="sourceCode"><pre class="sourceCode diff"><code class="sourceCode diff"> "dependencies": {
// ...
},
<span class="va">+ "lint-staged": {</span>
<span class="va">+ "src/**/*.{js,jsx,json,css}": [</span>
<span class="va">+ "prettier --single-quote --write",</span>
<span class="va">+ "git add"</span>
<span class="va">+ ]</span>
<span class="va">+ },</span>
"scripts": {</code></pre></div>
<p>Now, whenever you make a commit, Prettier will format the changed files automatically. You can also run <code>./node_modules/.bin/prettier --single-quote --write "src/**/*.{js,jsx,json,css}"</code> to format your entire project for the first time.</p>
<p>Next you might want to integrate Prettier in your favorite editor. Read the section on <a href="https://prettier.io/docs/en/editors.html">Editor Integration</a> on the Prettier GitHub page.</p>
<h2 id="changing-the-page-title">Changing the Page <code><title></code></h2>
<p>You can find the source HTML file in the <code>public</code> folder of the generated project. You may edit the <code><title></code> tag in it to change the title from “React App” to anything else.</p>
<p>Note that normally you wouldn’t edit files in the <code>public</code> folder very often. For example, <a href="#adding-a-stylesheet">adding a stylesheet</a> is done without touching the HTML.</p>
<p>If you need to dynamically update the page title based on the content, you can use the browser <a href="https://developer.mozilla.org/en-US/docs/Web/API/Document/title"><code>document.title</code></a> API. For more complex scenarios when you want to change the title from React components, you can use <a href="https://github.com/nfl/react-helmet">React Helmet</a>, a third party library.</p>
<p>If you use a custom server for your app in production and want to modify the title before it gets sent to the browser, you can follow advice in <a href="#generating-dynamic-meta-tags-on-the-server">this section</a>. Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is covered <a href="#pre-rendering-into-static-html-files">here</a>.</p>
<h2 id="installing-a-dependency">Installing a Dependency</h2>
<p>The generated project includes React and ReactDOM as dependencies. It also includes a set of scripts used by Create React App as a development dependency. You may install other dependencies (for example, React Router) with <code>npm</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save react-router</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add react-router</code></pre></div>
<p>This works for any library, not just <code>react-router</code>.</p>
<h2 id="importing-a-component">Importing a Component</h2>
<p>This project setup supports ES6 modules thanks to Babel.<br> While you can still use <code>require()</code> and <code>module.exports</code>, we encourage you to use <a href="http://exploringjs.com/es6/ch_modules.html"><code>import</code> and <code>export</code></a> instead.</p>
<p>For example:</p>
<h3 id="button.js"><code>Button.js</code></h3>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React<span class="op">,</span> <span class="op">{</span> Component <span class="op">}</span> <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="kw">class</span> Button <span class="kw">extends</span> Component <span class="op">{</span>
<span class="at">render</span>() <span class="op">{</span>
<span class="co">// ...</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="im">export</span> <span class="im">default</span> Button<span class="op">;</span> <span class="co">// Don’t forget to use export default!</span></code></pre></div>
<h3 id="dangerbutton.js"><code>DangerButton.js</code></h3>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React<span class="op">,</span> <span class="op">{</span> Component <span class="op">}</span> <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> Button <span class="im">from</span> <span class="st">'./Button'</span><span class="op">;</span> <span class="co">// Import a component from another file</span>
<span class="kw">class</span> DangerButton <span class="kw">extends</span> Component <span class="op">{</span>
<span class="at">render</span>() <span class="op">{</span>
<span class="cf">return</span> <span class="op"><</span>Button color<span class="op">=</span><span class="st">"red"</span> <span class="ss">/>;</span>
<span class="ss"> }</span>
<span class="ss">}</span>
<span class="ss">export default DangerButton;</span></code></pre></div>
<p>Be aware of the <a href="http://stackoverflow.com/questions/36795819/react-native-es-6-when-should-i-use-curly-braces-for-import/36796281#36796281">difference between default and named exports</a>. It is a common source of mistakes.</p>
<p>We suggest that you stick to using default imports and exports when a module only exports a single thing (for example, a component). That’s what you get when you use <code>export default Button</code> and <code>import Button from './Button'</code>.</p>
<p>Named exports are useful for utility modules that export several functions. A module may have at most one default export and as many named exports as you like.</p>
<p>Learn more about ES6 modules:</p>
<ul>
<li><a href="http://stackoverflow.com/questions/36795819/react-native-es-6-when-should-i-use-curly-braces-for-import/36796281#36796281">When to use the curly braces?</a></li>
<li><a href="http://exploringjs.com/es6/ch_modules.html">Exploring ES6: Modules</a></li>
<li><a href="https://leanpub.com/understandinges6/read#leanpub-auto-encapsulating-code-with-modules">Understanding ES6: Modules</a></li>
</ul>
<h2 id="code-splitting">Code Splitting</h2>
<p>Instead of downloading the entire app before users can use it, code splitting allows you to split your code into small chunks which you can then load on demand.</p>
<p>This project setup supports code splitting via <a href="http://2ality.com/2017/01/import-operator.html#loading-code-on-demand">dynamic <code>import()</code></a>. Its <a href="https://github.com/tc39/proposal-dynamic-import">proposal</a> is in stage 3. The <code>import()</code> function-like form takes the module name as an argument and returns a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise"><code>Promise</code></a> which always resolves to the namespace object of the module.</p>
<p>Here is an example:</p>
<h3 id="modulea.js"><code>moduleA.js</code></h3>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="kw">const</span> moduleA <span class="op">=</span> <span class="st">'Hello'</span><span class="op">;</span>
<span class="im">export</span> <span class="op">{</span> moduleA <span class="op">};</span></code></pre></div>
<h3 id="app.js"><code>App.js</code></h3>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React<span class="op">,</span> <span class="op">{</span> Component <span class="op">}</span> <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="kw">class</span> App <span class="kw">extends</span> Component <span class="op">{</span>
handleClick <span class="op">=</span> () <span class="op">=></span> <span class="op">{</span>
<span class="im">import</span>(<span class="st">'./moduleA'</span>)
.<span class="at">then</span>((<span class="op">{</span> moduleA <span class="op">}</span>) <span class="op">=></span> <span class="op">{</span>
<span class="co">// Use moduleA</span>
<span class="op">}</span>)
.<span class="at">catch</span>(err <span class="op">=></span> <span class="op">{</span>
<span class="co">// Handle failure</span>
<span class="op">}</span>)<span class="op">;</span>
<span class="op">};</span>
<span class="at">render</span>() <span class="op">{</span>
<span class="cf">return</span> (
<span class="op"><</span>div<span class="op">></span>
<span class="op"><</span>button onClick<span class="op">={</span><span class="kw">this</span>.<span class="at">handleClick</span><span class="op">}></span>Load<span class="op"><</span><span class="ss">/button></span>
<span class="ss"> </div</span><span class="op">></span>
)<span class="op">;</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="im">export</span> <span class="im">default</span> App<span class="op">;</span></code></pre></div>
<p>This will make <code>moduleA.js</code> and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.</p>
<p>You can also use it with <code>async</code> / <code>await</code> syntax if you prefer it.</p>
<h3 id="with-react-router">With React Router</h3>
<p>If you are using React Router check out <a href="http://serverless-stack.com/chapters/code-splitting-in-create-react-app.html">this tutorial</a> on how to use code splitting with it. You can find the companion GitHub repository <a href="https://github.com/AnomalyInnovations/serverless-stack-demo-client/tree/code-splitting-in-create-react-app">here</a>.</p>
<p>Also check out the <a href="https://reactjs.org/docs/code-splitting.html">Code Splitting</a> section in React documentation.</p>
<h2 id="adding-a-stylesheet">Adding a Stylesheet</h2>
<p>This project setup uses <a href="https://webpack.js.org/">Webpack</a> for handling all assets. Webpack offers a custom way of “extending” the concept of <code>import</code> beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need to <strong>import the CSS from the JavaScript file</strong>:</p>
<h3 id="button.css"><code>Button.css</code></h3>
<div class="sourceCode"><pre class="sourceCode css"><code class="sourceCode css"><span class="fu">.Button</span> {
<span class="kw">padding</span>: <span class="dv">20px</span>;
}</code></pre></div>
<h3 id="button.js-1"><code>Button.js</code></h3>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React<span class="op">,</span> <span class="op">{</span> Component <span class="op">}</span> <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> <span class="st">'./Button.css'</span><span class="op">;</span> <span class="co">// Tell Webpack that Button.js uses these styles</span>
<span class="kw">class</span> Button <span class="kw">extends</span> Component <span class="op">{</span>
<span class="at">render</span>() <span class="op">{</span>
<span class="co">// You can use them as regular CSS styles</span>
<span class="cf">return</span> <span class="op"><</span>div className<span class="op">=</span><span class="st">"Button"</span> <span class="ss">/>;</span>
<span class="ss"> }</span>
<span class="ss">}</span></code></pre></div>
<p><strong>This is not required for React</strong> but many people find this feature convenient. You can read about the benefits of this approach <a href="https://medium.com/seek-ui-engineering/block-element-modifying-your-javascript-components-d7f99fcab52b">here</a>. However you should be aware that this makes your code less portable to other build tools and environments than Webpack.</p>
<p>In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified <code>.css</code> file in the build output.</p>
<p>If you are concerned about using Webpack-specific semantics, you can put all your CSS right into <code>src/index.css</code>. It would still be imported from <code>src/index.js</code>, but you could always remove that import if you later migrate to a different build tool.</p>
<h2 id="post-processing-css">Post-Processing CSS</h2>
<p>This project setup minifies your CSS and adds vendor prefixes to it automatically through <a href="https://github.com/postcss/autoprefixer">Autoprefixer</a> so you don’t need to worry about it.</p>
<p>For example, this:</p>
<div class="sourceCode"><pre class="sourceCode css"><code class="sourceCode css"><span class="fu">.App</span> {
<span class="kw">display</span>: flex;
<span class="kw">flex-direction</span>: row;
<span class="kw">align-items</span>: <span class="dv">center</span>;
}</code></pre></div>
<p>becomes this:</p>
<div class="sourceCode"><pre class="sourceCode css"><code class="sourceCode css"><span class="fu">.App</span> {
<span class="kw">display</span>: -webkit-box;
<span class="kw">display</span>: -ms-flexbox;
<span class="kw">display</span>: flex;
-webkit-box-orient: horizontal;
-webkit-box-direction: <span class="dv">normal</span>;
-ms-flex-direction: row;
<span class="kw">flex-direction</span>: row;
-webkit-box-align: <span class="dv">center</span>;
-ms-flex-align: <span class="dv">center</span>;
<span class="kw">align-items</span>: <span class="dv">center</span>;
}</code></pre></div>
<p>If you need to disable autoprefixing for some reason, <a href="https://github.com/postcss/autoprefixer#disabling">follow this section</a>.</p>
<h2 id="adding-a-css-preprocessor-sass-less-etc.">Adding a CSS Preprocessor (Sass, Less etc.)</h2>
<p>Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a <code>.Button</code> CSS class in <code><AcceptButton></code> and <code><RejectButton></code> components, we recommend creating a <code><Button></code> component with its own <code>.Button</code> styles, that both <code><AcceptButton></code> and <code><RejectButton></code> can render (but <a href="https://facebook.github.io/react/docs/composition-vs-inheritance.html">not inherit</a>).</p>
<p>Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.</p>
<p>First, let’s install the command-line interface for Sass:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save node-sass-chokidar</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add node-sass-chokidar</code></pre></div>
<p>Then in <code>package.json</code>, add the following lines to <code>scripts</code>:</p>
<div class="sourceCode"><pre class="sourceCode diff"><code class="sourceCode diff"> "scripts": {
<span class="va">+ "build-css": "node-sass-chokidar src/ -o src/",</span>
<span class="va">+ "watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",</span>
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test --env=jsdom",</code></pre></div>
<blockquote>
<p>Note: To use a different preprocessor, replace <code>build-css</code> and <code>watch-css</code> commands according to your preprocessor’s documentation.</p>
</blockquote>
<p>Now you can rename <code>src/App.css</code> to <code>src/App.scss</code> and run <code>npm run watch-css</code>. The watcher will find every Sass file in <code>src</code> subdirectories, and create a corresponding CSS file next to it, in our case overwriting <code>src/App.css</code>. Since <code>src/App.js</code> still imports <code>src/App.css</code>, the styles become a part of your application. You can now edit <code>src/App.scss</code>, and <code>src/App.css</code> will be regenerated.</p>
<p>To share variables between Sass files, you can use Sass imports. For example, <code>src/App.scss</code> and other component style files could include <code>@import "./shared.scss";</code> with variable definitions.</p>
<p>To enable importing files without using relative paths, you can add the <code>--include-path</code> option to the command in <code>package.json</code>.</p>
<pre><code>"build-css": "node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/",
"watch-css": "npm run build-css && node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/ --watch --recursive",</code></pre>
<p>This will allow you to do imports like</p>
<pre class="scss"><code>@import 'styles/_colors.scss'; // assuming a styles directory under src/
@import 'nprogress/nprogress'; // importing a css file from the nprogress node module</code></pre>
<p>At this point you might want to remove all CSS files from the source control, and add <code>src/**/*.css</code> to your <code>.gitignore</code> file. It is generally a good practice to keep the build products outside of the source control.</p>
<p>As a final step, you may find it convenient to run <code>watch-css</code> automatically with <code>npm start</code>, and run <code>build-css</code> as a part of <code>npm run build</code>. You can use the <code>&&</code> operator to execute two scripts sequentially. However, there is no cross-platform way to run two scripts in parallel, so we will install a package for this:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save npm-run-all</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add npm-run-all</code></pre></div>
<p>Then we can change <code>start</code> and <code>build</code> scripts to include the CSS preprocessor commands:</p>
<div class="sourceCode"><pre class="sourceCode diff"><code class="sourceCode diff"> "scripts": {
"build-css": "node-sass-chokidar src/ -o src/",
"watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",
<span class="st">- "start": "react-scripts start",</span>
<span class="st">- "build": "react-scripts build",</span>
<span class="va">+ "start-js": "react-scripts start",</span>
<span class="va">+ "start": "npm-run-all -p watch-css start-js",</span>
<span class="va">+ "build-js": "react-scripts build",</span>
<span class="va">+ "build": "npm-run-all build-css build-js",</span>
"test": "react-scripts test --env=jsdom",
"eject": "react-scripts eject"
}</code></pre></div>
<p>Now running <code>npm start</code> and <code>npm run build</code> also builds Sass files.</p>
<p><strong>Why <code>node-sass-chokidar</code>?</strong></p>
<p><code>node-sass</code> has been reported as having the following issues:</p>
<ul>
<li><p><code>node-sass --watch</code> has been reported to have <em>performance issues</em> in certain conditions when used in a virtual machine or with docker.</p></li>
<li><p>Infinite styles compiling <a href="https://github.com/facebookincubator/create-react-app/issues/1939">#1939</a></p></li>
<li><p><code>node-sass</code> has been reported as having issues with detecting new files in a directory <a href="https://github.com/sass/node-sass/issues/1891">#1891</a></p></li>
</ul>
<p><code>node-sass-chokidar</code> is used here as it addresses these issues.</p>
<h2 id="adding-images-fonts-and-files">Adding Images, Fonts, and Files</h2>
<p>With Webpack, using static assets like images and fonts works similarly to CSS.</p>
<p>You can <strong><code>import</code> a file right in a JavaScript module</strong>. This tells Webpack to include that file in the bundle. Unlike CSS imports, importing a file gives you a string value. This value is the final path you can reference in your code, e.g. as the <code>src</code> attribute of an image or the <code>href</code> of a link to a PDF.</p>
<p>To reduce the number of requests to the server, importing images that are less than 10,000 bytes returns a <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs">data URI</a> instead of a path. This applies to the following file extensions: bmp, gif, jpg, jpeg, and png. SVG files are excluded due to <a href="https://github.com/facebookincubator/create-react-app/issues/1153">#1153</a>.</p>
<p>Here is an example:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> logo <span class="im">from</span> <span class="st">'./logo.png'</span><span class="op">;</span> <span class="co">// Tell Webpack this JS file uses this image</span>
<span class="va">console</span>.<span class="at">log</span>(logo)<span class="op">;</span> <span class="co">// /logo.84287d09.png</span>
<span class="kw">function</span> <span class="at">Header</span>() <span class="op">{</span>
<span class="co">// Import result is the URL of your image</span>
<span class="cf">return</span> <span class="op"><</span>img src<span class="op">={</span>logo<span class="op">}</span> alt<span class="op">=</span><span class="st">"Logo"</span> <span class="ss">/>;</span>
<span class="ss">}</span>
<span class="ss">export default Header;</span></code></pre></div>
<p>This ensures that when the project is built, Webpack will correctly move the images into the build folder, and provide us with correct paths.</p>
<p>This works in CSS too:</p>
<div class="sourceCode"><pre class="sourceCode css"><code class="sourceCode css"><span class="fu">.Logo</span> {
<span class="kw">background-image</span>: <span class="fu">url</span>(<span class="dv">.</span>/logo<span class="dv">.</span>png);
}</code></pre></div>
<p>Webpack finds all relative module references in CSS (they start with <code>./</code>) and replaces them with the final paths from the compiled bundle. If you make a typo or accidentally delete an important file, you will see a compilation error, just like when you import a non-existent JavaScript module. The final filenames in the compiled bundle are generated by Webpack from content hashes. If the file content changes in the future, Webpack will give it a different name in production so you don’t need to worry about long-term caching of assets.</p>
<p>Please be advised that this is also a custom feature of Webpack.</p>
<p><strong>It is not required for React</strong> but many people enjoy it (and React Native uses a similar mechanism for images).<br> An alternative way of handling static assets is described in the next section.</p>
<h2 id="using-the-public-folder">Using the <code>public</code> Folder</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.5.0</code> and higher.</p>
</blockquote>
<h3 id="changing-the-html">Changing the HTML</h3>
<p>The <code>public</code> folder contains the HTML file so you can tweak it, for example, to <a href="#changing-the-page-title">set the page title</a>. The <code><script></code> tag with the compiled code will be added to it automatically during the build process.</p>
<h3 id="adding-assets-outside-of-the-module-system">Adding Assets Outside of the Module System</h3>
<p>You can also add other assets to the <code>public</code> folder.</p>
<p>Note that we normally encourage you to <code>import</code> assets in JavaScript files instead. For example, see the sections on <a href="#adding-a-stylesheet">adding a stylesheet</a> and <a href="#adding-images-fonts-and-files">adding images and fonts</a>. This mechanism provides a number of benefits:</p>
<ul>
<li>Scripts and stylesheets get minified and bundled together to avoid extra network requests.</li>
<li>Missing files cause compilation errors instead of 404 errors for your users.</li>
<li>Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.</li>
</ul>
<p>However there is an <strong>escape hatch</strong> that you can use to add an asset outside of the module system.</p>
<p>If you put a file into the <code>public</code> folder, it will <strong>not</strong> be processed by Webpack. Instead it will be copied into the build folder untouched. To reference assets in the <code>public</code> folder, you need to use a special variable called <code>PUBLIC_URL</code>.</p>
<p>Inside <code>index.html</code>, you can use it like this:</p>
<div class="sourceCode"><pre class="sourceCode html"><code class="sourceCode html"><span class="kw"><link</span><span class="ot"> rel=</span><span class="st">"shortcut icon"</span><span class="ot"> href=</span><span class="st">"%PUBLIC_URL%/favicon.ico"</span><span class="kw">></span></code></pre></div>
<p>Only files inside the <code>public</code> folder will be accessible by <code>%PUBLIC_URL%</code> prefix. If you need to use a file from <code>src</code> or <code>node_modules</code>, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.</p>
<p>When you run <code>npm run build</code>, Create React App will substitute <code>%PUBLIC_URL%</code> with a correct absolute path so your project works even if you use client-side routing or host it at a non-root URL.</p>
<p>In JavaScript code, you can use <code>process.env.PUBLIC_URL</code> for similar purposes:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="at">render</span>() <span class="op">{</span>
<span class="co">// Note: this is an escape hatch and should be used sparingly!</span>
<span class="co">// Normally we recommend using `import` for getting asset URLs</span>
<span class="co">// as described in “Adding Images and Fonts” above this section.</span>
<span class="cf">return</span> <span class="op"><</span>img src<span class="op">={</span><span class="va">process</span>.<span class="va">env</span>.<span class="at">PUBLIC_URL</span> <span class="op">+</span> <span class="st">'/img/logo.png'</span><span class="op">}</span> <span class="ss">/>;</span>
<span class="ss">}</span></code></pre></div>
<p>Keep in mind the downsides of this approach:</p>
<ul>
<li>None of the files in <code>public</code> folder get post-processed or minified.</li>
<li>Missing files will not be called at compilation time, and will cause 404 errors for your users.</li>
<li>Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.</li>
</ul>
<h3 id="when-to-use-the-public-folder">When to Use the <code>public</code> Folder</h3>
<p>Normally we recommend importing <a href="#adding-a-stylesheet">stylesheets</a>, <a href="#adding-images-fonts-and-files">images, and fonts</a> from JavaScript. The <code>public</code> folder is useful as a workaround for a number of less common cases:</p>
<ul>
<li>You need a file with a specific name in the build output, such as <a href="https://developer.mozilla.org/en-US/docs/Web/Manifest"><code>manifest.webmanifest</code></a>.</li>
<li>You have thousands of images and need to dynamically reference their paths.</li>
<li>You want to include a small script like <a href="http://github.hubspot.com/pace/docs/welcome/"><code>pace.js</code></a> outside of the bundled code.</li>
<li>Some library may be incompatible with Webpack and you have no other option but to include it as a <code><script></code> tag.</li>
</ul>
<p>Note that if you add a <code><script></code> that declares global variables, you also need to read the next section on using them.</p>
<h2 id="using-global-variables">Using Global Variables</h2>
<p>When you include a script in the HTML file that defines global variables and try to use one of these variables in the code, the linter will complain because it cannot see the definition of the variable.</p>
<p>You can avoid this by reading the global variable explicitly from the <code>window</code> object, for example:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="kw">const</span> $ <span class="op">=</span> <span class="va">window</span>.<span class="at">$</span><span class="op">;</span></code></pre></div>
<p>This makes it obvious you are using a global variable intentionally rather than because of a typo.</p>
<p>Alternatively, you can force the linter to ignore any line by adding <code>// eslint-disable-line</code> after it.</p>
<h2 id="adding-bootstrap">Adding Bootstrap</h2>
<p>You don’t have to use <a href="https://react-bootstrap.github.io">React Bootstrap</a> together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:</p>
<p>Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save react-bootstrap bootstrap@3</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add react-bootstrap bootstrap@3</code></pre></div>
<p>Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your <code>src/index.js</code> file:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> <span class="st">'bootstrap/dist/css/bootstrap.css'</span><span class="op">;</span>
<span class="im">import</span> <span class="st">'bootstrap/dist/css/bootstrap-theme.css'</span><span class="op">;</span>
<span class="co">// Put any other imports below so that CSS from your</span>
<span class="co">// components takes precedence over default styles.</span></code></pre></div>
<p>Import required React Bootstrap components within <code>src/App.js</code> file or your custom component files:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> <span class="op">{</span> Navbar<span class="op">,</span> Jumbotron<span class="op">,</span> Button <span class="op">}</span> <span class="im">from</span> <span class="st">'react-bootstrap'</span><span class="op">;</span></code></pre></div>
<p>Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example <a href="https://gist.githubusercontent.com/gaearon/85d8c067f6af1e56277c82d19fd4da7b/raw/6158dd991b67284e9fc8d70b9d973efe87659d72/App.js"><code>App.js</code></a> redone using React Bootstrap.</p>
<h3 id="using-a-custom-theme">Using a Custom Theme</h3>
<p>Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br> We suggest the following approach:</p>
<ul>
<li>Create a new package that depends on the package you wish to customize, e.g. Bootstrap.</li>
<li>Add the necessary build steps to tweak the theme, and publish your package on npm.</li>
<li>Install your own theme npm package as a dependency of your app.</li>
</ul>
<p>Here is an example of adding a <a href="https://medium.com/@tacomanator/customizing-create-react-app-aa9ffb88165">customized Bootstrap</a> that follows these steps.</p>
<h2 id="adding-flow">Adding Flow</h2>
<p>Flow is a static type checker that helps you write code with fewer bugs. Check out this <a href="https://medium.com/@preethikasireddy/why-use-static-types-in-javascript-part-1-8382da1e0adb">introduction to using static types in JavaScript</a> if you are new to this concept.</p>
<p>Recent versions of <a href="http://flowtype.org/">Flow</a> work with Create React App projects out of the box.</p>
<p>To add Flow to a Create React App project, follow these steps:</p>
<ol style="list-style-type: decimal">
<li>Run <code>npm install --save flow-bin</code> (or <code>yarn add flow-bin</code>).</li>
<li>Add <code>"flow": "flow"</code> to the <code>scripts</code> section of your <code>package.json</code>.</li>
<li>Run <code>npm run flow init</code> (or <code>yarn flow init</code>) to create a <a href="https://flowtype.org/docs/advanced-configuration.html"><code>.flowconfig</code> file</a> in the root directory.</li>
<li>Add <code>// @flow</code> to any files you want to type check (for example, to <code>src/App.js</code>).</li>
</ol>
<p>Now you can run <code>npm run flow</code> (or <code>yarn flow</code>) to check the files for type errors. You can optionally use an IDE like <a href="https://nuclide.io/docs/languages/flow/">Nuclide</a> for a better integrated experience. In the future we plan to integrate it into Create React App even more closely.</p>
<p>To learn more about Flow, check out <a href="https://flowtype.org/">its documentation</a>.</p>
<h2 id="adding-a-router">Adding a Router</h2>
<p>Create React App doesn't prescribe a specific routing solution, but <a href="https://reacttraining.com/react-router/">React Router</a> is the most popular one.</p>
<p>To add it, run:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save react-router-dom</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add react-router-dom</code></pre></div>
<p>To try it, delete all the code in <code>src/App.js</code> and replace it with any of the examples on its website. The <a href="https://reacttraining.com/react-router/web/example/basic">Basic Example</a> is a good place to get started.</p>
<p>Note that <a href="#serving-apps-with-client-side-routing">you may need to configure your production server to support client-side routing</a> before deploying your app.</p>
<h2 id="adding-custom-environment-variables">Adding Custom Environment Variables</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.2.3</code> and higher.</p>
</blockquote>
<p>Your project can consume variables declared in your environment as if they were declared locally in your JS files. By default you will have <code>NODE_ENV</code> defined for you, and any other environment variables starting with <code>REACT_APP_</code>.</p>
<p><strong>The environment variables are embedded during the build time</strong>. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, just like <a href="#injecting-data-from-the-server-into-the-page">described here</a>. Alternatively you can rebuild the app on the server anytime you change them.</p>
<blockquote>
<p>Note: You must create custom environment variables beginning with <code>REACT_APP_</code>. Any other variables except <code>NODE_ENV</code> will be ignored to avoid accidentally <a href="https://github.com/facebookincubator/create-react-app/issues/865#issuecomment-252199527">exposing a private key on the machine that could have the same name</a>. Changing any environment variables will require you to restart the development server if it is running.</p>
</blockquote>
<p>These environment variables will be defined for you on <code>process.env</code>. For example, having an environment variable named <code>REACT_APP_SECRET_CODE</code> will be exposed in your JS as <code>process.env.REACT_APP_SECRET_CODE</code>.</p>
<p>There is also a special built-in environment variable called <code>NODE_ENV</code>. You can read it from <code>process.env.NODE_ENV</code>. When you run <code>npm start</code>, it is always equal to <code>'development'</code>, when you run <code>npm test</code> it is always equal to <code>'test'</code>, and when you run <code>npm run build</code> to make a production bundle, it is always equal to <code>'production'</code>. <strong>You cannot override <code>NODE_ENV</code> manually.</strong> This prevents developers from accidentally deploying a slow development build to production.</p>
<p>These environment variables can be useful for displaying information conditionally based on where the project is deployed or consuming sensitive data that lives outside of version control.</p>
<p>First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined in the environment inside a <code><form></code>:</p>
<pre class="jsx"><code>render() {
return (
<div>
<small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
<form>
<input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
</form>
</div>
);
}</code></pre>
<p>During the build, <code>process.env.REACT_APP_SECRET_CODE</code> will be replaced with the current value of the <code>REACT_APP_SECRET_CODE</code> environment variable. Remember that the <code>NODE_ENV</code> variable will be set for you automatically.</p>
<p>When you load the app in the browser and inspect the <code><input></code>, you will see its value set to <code>abcdef</code>, and the bold text will show the environment provided when using <code>npm start</code>:</p>
<div class="sourceCode"><pre class="sourceCode html"><code class="sourceCode html"><span class="kw"><div></span>
<span class="kw"><small></span>You are running this application in <span class="kw"><b></span>development<span class="kw"></b></span> mode.<span class="kw"></small></span>
<span class="kw"><form></span>
<span class="kw"><input</span><span class="ot"> type=</span><span class="st">"hidden"</span><span class="ot"> value=</span><span class="st">"abcdef"</span> <span class="kw">/></span>
<span class="kw"></form></span>
<span class="kw"></div></span></code></pre></div>
<p>The above form is looking for a variable called <code>REACT_APP_SECRET_CODE</code> from the environment. In order to consume this value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in a <code>.env</code> file. Both of these ways are described in the next few sections.</p>
<p>Having access to the <code>NODE_ENV</code> is also useful for performing actions conditionally:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="cf">if</span> (<span class="va">process</span>.<span class="va">env</span>.<span class="at">NODE_ENV</span> <span class="op">!==</span> <span class="st">'production'</span>) <span class="op">{</span>
<span class="va">analytics</span>.<span class="at">disable</span>()<span class="op">;</span>
<span class="op">}</span></code></pre></div>
<p>When you compile the app with <code>npm run build</code>, the minification step will strip out this condition, and the resulting bundle will be smaller.</p>
<h3 id="referencing-environment-variables-in-the-html">Referencing Environment Variables in the HTML</h3>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.9.0</code> and higher.</p>
</blockquote>
<p>You can also access the environment variables starting with <code>REACT_APP_</code> in the <code>public/index.html</code>. For example:</p>
<div class="sourceCode"><pre class="sourceCode html"><code class="sourceCode html"><span class="kw"><title></span>%REACT_APP_WEBSITE_NAME%<span class="kw"></title></span></code></pre></div>
<p>Note that the caveats from the above section apply:</p>
<ul>
<li>Apart from a few built-in variables (<code>NODE_ENV</code> and <code>PUBLIC_URL</code>), variable names must start with <code>REACT_APP_</code> to work.</li>
<li>The environment variables are injected at build time. If you need to inject them at runtime, <a href="#generating-dynamic-meta-tags-on-the-server">follow this approach instead</a>.</li>
</ul>
<h3 id="adding-temporary-environment-variables-in-your-shell">Adding Temporary Environment Variables In Your Shell</h3>
<p>Defining environment variables can vary between OSes. It’s also important to know that this manner is temporary for the life of the shell session.</p>
<h4 id="windows-cmd.exe">Windows (cmd.exe)</h4>
<pre class="cmd"><code>set "REACT_APP_SECRET_CODE=abcdef" && npm start</code></pre>
<p>(Note: Quotes around the variable assignment are required to avoid a trailing whitespace.)</p>
<h4 id="windows-powershell">Windows (Powershell)</h4>
<pre class="powershell"><code>($env:REACT_APP_SECRET_CODE = "abcdef") -and (npm start)</code></pre>
<h4 id="linux-macos-bash">Linux, macOS (Bash)</h4>
<div class="sourceCode"><pre class="sourceCode bash"><code class="sourceCode bash"><span class="va">REACT_APP_SECRET_CODE=</span>abcdef <span class="ex">npm</span> start</code></pre></div>
<h3 id="adding-development-environment-variables-in-.env">Adding Development Environment Variables In <code>.env</code></h3>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.5.0</code> and higher.</p>
</blockquote>
<p>To define permanent environment variables, create a file called <code>.env</code> in the root of your project:</p>
<pre><code>REACT_APP_SECRET_CODE=abcdef</code></pre>
<blockquote>
<p>Note: You must create custom environment variables beginning with <code>REACT_APP_</code>. Any other variables except <code>NODE_ENV</code> will be ignored to avoid <a href="https://github.com/facebookincubator/create-react-app/issues/865#issuecomment-252199527">accidentally exposing a private key on the machine that could have the same name</a>. Changing any environment variables will require you to restart the development server if it is running.</p>
</blockquote>
<p><code>.env</code> files <strong>should be</strong> checked into source control (with the exclusion of <code>.env*.local</code>).</p>
<h4 id="what-other-.env-files-can-be-used">What other <code>.env</code> files can be used?</h4>
<blockquote>
<p>Note: this feature is <strong>available with <code>react-scripts@1.0.0</code> and higher</strong>.</p>
</blockquote>
<ul>
<li><code>.env</code>: Default.</li>
<li><code>.env.local</code>: Local overrides. <strong>This file is loaded for all environments except test.</strong></li>
<li><code>.env.development</code>, <code>.env.test</code>, <code>.env.production</code>: Environment-specific settings.</li>
<li><code>.env.development.local</code>, <code>.env.test.local</code>, <code>.env.production.local</code>: Local overrides of environment-specific settings.</li>
</ul>
<p>Files on the left have more priority than files on the right:</p>
<ul>
<li><code>npm start</code>: <code>.env.development.local</code>, <code>.env.development</code>, <code>.env.local</code>, <code>.env</code></li>
<li><code>npm run build</code>: <code>.env.production.local</code>, <code>.env.production</code>, <code>.env.local</code>, <code>.env</code></li>
<li><code>npm test</code>: <code>.env.test.local</code>, <code>.env.test</code>, <code>.env</code> (note <code>.env.local</code> is missing)</li>
</ul>
<p>These variables will act as the defaults if the machine does not explicitly set them.<br> Please refer to the <a href="https://github.com/motdotla/dotenv">dotenv documentation</a> for more details.</p>
<blockquote>
<p>Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need these defined as well. Consult their documentation how to do this. For example, see the documentation for <a href="https://docs.travis-ci.com/user/environment-variables/">Travis CI</a> or <a href="https://devcenter.heroku.com/articles/config-vars">Heroku</a>.</p>
</blockquote>
<h4 id="expanding-environment-variables-in-.env">Expanding Environment Variables In <code>.env</code></h4>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@1.1.0</code> and higher.</p>
</blockquote>
<p>Expand variables already on your machine for use in your <code>.env</code> file (using <a href="https://github.com/motdotla/dotenv-expand">dotenv-expand</a>).</p>
<p>For example, to get the environment variable <code>npm_package_version</code>:</p>
<pre><code>REACT_APP_VERSION=$npm_package_version
# also works:
# REACT_APP_VERSION=${npm_package_version}</code></pre>
<p>Or expand variables local to the current <code>.env</code> file:</p>
<pre><code>DOMAIN=www.example.com
REACT_APP_FOO=$DOMAIN/foo
REACT_APP_BAR=$DOMAIN/bar</code></pre>
<h2 id="can-i-use-decorators">Can I Use Decorators?</h2>
<p>Many popular libraries use <a href="https://medium.com/google-developers/exploring-es7-decorators-76ecb65fb841">decorators</a> in their documentation.<br> Create React App doesn’t support decorator syntax at the moment because:</p>
<ul>
<li>It is an experimental proposal and is subject to change.</li>
<li>The current specification version is not officially supported by Babel.</li>
<li>If the specification changes, we won’t be able to write a codemod because we don’t use them internally at Facebook.</li>
</ul>
<p>However in many cases you can rewrite decorator-based code without decorators just as fine.<br> Please refer to these two threads for reference:</p>
<ul>
<li><a href="https://github.com/facebookincubator/create-react-app/issues/214">#214</a></li>
<li><a href="https://github.com/facebookincubator/create-react-app/issues/411">#411</a></li>
</ul>
<p>Create React App will add decorator support when the specification advances to a stable stage.</p>
<h2 id="fetching-data-with-ajax-requests">Fetching Data with AJAX Requests</h2>
<p>React doesn't prescribe a specific approach to data fetching, but people commonly use either a library like <a href="https://github.com/axios/axios">axios</a> or the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API"><code>fetch()</code> API</a> provided by the browser. Conveniently, Create React App includes a polyfill for <code>fetch()</code> so you can use it without worrying about the browser support.</p>
<p>The global <code>fetch</code> function allows to easily makes AJAX requests. It takes in a URL as an input and returns a <code>Promise</code> that resolves to a <code>Response</code> object. You can find more information about <code>fetch</code> <a href="https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch">here</a>.</p>
<p>This project also includes a <a href="https://github.com/then/promise">Promise polyfill</a> which provides a full implementation of Promises/A+. A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises <a href="https://www.promisejs.org/">here</a> and <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise">here</a>. Both axios and <code>fetch()</code> use Promises under the hood. You can also use the <a href="https://davidwalsh.name/async-await"><code>async / await</code></a> syntax to reduce the callback nesting.</p>
<p>You can learn more about making AJAX requests from React components in <a href="https://reactjs.org/docs/faq-ajax.html">the FAQ entry on the React website</a>.</p>
<h2 id="integrating-with-an-api-backend">Integrating with an API Backend</h2>
<p>These tutorials will help you to integrate your app with an API backend running on another port, using <code>fetch()</code> to access it.</p>
<h3 id="node">Node</h3>
<p>Check out <a href="https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/">this tutorial</a>. You can find the companion GitHub repository <a href="https://github.com/fullstackreact/food-lookup-demo">here</a>.</p>
<h3 id="ruby-on-rails">Ruby on Rails</h3>
<p>Check out <a href="https://www.fullstackreact.com/articles/how-to-get-create-react-app-to-work-with-your-rails-api/">this tutorial</a>. You can find the companion GitHub repository <a href="https://github.com/fullstackreact/food-lookup-demo-rails">here</a>.</p>
<h2 id="proxying-api-requests-in-development">Proxying API Requests in Development</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.2.3</code> and higher.</p>
</blockquote>
<p>People often serve the front-end React app from the same host and port as their backend implementation.<br> For example, a production setup might look like this after the app is deployed:</p>
<pre><code>/ - static server returns index.html with React app
/todos - static server returns index.html with React app
/api/todos - server handles any /api/* requests using the backend implementation</code></pre>
<p>Such setup is <strong>not</strong> required. However, if you <strong>do</strong> have a setup like this, it is convenient to write requests like <code>fetch('/api/todos')</code> without worrying about redirecting them to another host or port during development.</p>
<p>To tell the development server to proxy any unknown requests to your API server in development, add a <code>proxy</code> field to your <code>package.json</code>, for example:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"> <span class="st">"proxy"</span><span class="op">:</span> <span class="st">"http://localhost:4000"</span><span class="op">,</span></code></pre></div>
<p>This way, when you <code>fetch('/api/todos')</code> in development, the development server will recognize that it’s not a static asset, and will proxy your request to <code>http://localhost:4000/api/todos</code> as a fallback. The development server will <strong>only</strong> attempt to send requests without <code>text/html</code> in its <code>Accept</code> header to the proxy.</p>
<p>Conveniently, this avoids <a href="http://stackoverflow.com/questions/21854516/understanding-ajax-cors-and-security-considerations">CORS issues</a> and error messages like this in development:</p>
<pre><code>Fetch API cannot load http://localhost:4000/api/todos. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.</code></pre>
<p>Keep in mind that <code>proxy</code> only has effect in development (with <code>npm start</code>), and it is up to you to ensure that URLs like <code>/api/todos</code> point to the right thing in production. You don’t have to use the <code>/api</code> prefix. Any unrecognized request without a <code>text/html</code> accept header will be redirected to the specified <code>proxy</code>.</p>
<p>The <code>proxy</code> option supports HTTP, HTTPS and WebSocket connections.<br> If the <code>proxy</code> option is <strong>not</strong> flexible enough for you, alternatively you can:</p>
<ul>
<li><a href="#configuring-the-proxy-manually">Configure the proxy yourself</a></li>
<li>Enable CORS on your server (<a href="http://enable-cors.org/server_expressjs.html">here’s how to do it for Express</a>).</li>
<li>Use <a href="#adding-custom-environment-variables">environment variables</a> to inject the right server host and port into your app.</li>
</ul>
<h3 id="invalid-host-header-errors-after-configuring-proxy">"Invalid Host Header" Errors After Configuring Proxy</h3>
<p>When you enable the <code>proxy</code> option, you opt into a more strict set of host checks. This is necessary because leaving the backend open to remote hosts makes your computer vulnerable to DNS rebinding attacks. The issue is explained in <a href="https://medium.com/webpack/webpack-dev-server-middleware-security-issues-1489d950874a">this article</a> and <a href="https://github.com/webpack/webpack-dev-server/issues/887">this issue</a>.</p>
<p>This shouldn’t affect you when developing on <code>localhost</code>, but if you develop remotely like <a href="https://github.com/facebookincubator/create-react-app/issues/2271">described here</a>, you will see this error in the browser after enabling the <code>proxy</code> option:</p>
<blockquote>
<p>Invalid Host header</p>
</blockquote>
<p>To work around it, you can specify your public development host in a file called <code>.env.development</code> in the root of your project:</p>
<pre><code>HOST=mypublicdevhost.com</code></pre>
<p>If you restart the development server now and load the app from the specified host, it should work.</p>
<p>If you are still having issues or if you’re using a more exotic environment like a cloud editor, you can bypass the host check completely by adding a line to <code>.env.development.local</code>. <strong>Note that this is dangerous and exposes your machine to remote code execution from malicious websites:</strong></p>
<pre><code># NOTE: THIS IS DANGEROUS!
# It exposes your machine to attacks from the websites you visit.
DANGEROUSLY_DISABLE_HOST_CHECK=true</code></pre>
<p>We don’t recommend this approach.</p>
<h3 id="configuring-the-proxy-manually">Configuring the Proxy Manually</h3>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@1.0.0</code> and higher.</p>
</blockquote>
<p>If the <code>proxy</code> option is <strong>not</strong> flexible enough for you, you can specify an object in the following form (in <code>package.json</code>).<br> You may also specify any configuration value <a href="https://github.com/chimurai/http-proxy-middleware#options"><code>http-proxy-middleware</code></a> or <a href="https://github.com/nodejitsu/node-http-proxy#options"><code>http-proxy</code></a> supports.</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="op">{</span>
<span class="co">// ...</span>
<span class="st">"proxy"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"/api"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"<url>"</span><span class="op">,</span>
<span class="st">"ws"</span><span class="op">:</span> <span class="kw">true</span>
<span class="co">// ...</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="co">// ...</span>
<span class="op">}</span></code></pre></div>
<p>All requests matching this path will be proxies, no exceptions. This includes requests for <code>text/html</code>, which the standard <code>proxy</code> option does not proxy.</p>
<p>If you need to specify multiple proxies, you may do so by specifying additional entries. Matches are regular expressions, so that you can use a regexp to match multiple paths.</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="op">{</span>
<span class="co">// ...</span>
<span class="st">"proxy"</span><span class="op">:</span> <span class="op">{</span>
<span class="co">// Matches any request starting with /api</span>
<span class="st">"/api"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"<url_1>"</span><span class="op">,</span>
<span class="st">"ws"</span><span class="op">:</span> <span class="kw">true</span>
<span class="co">// ...</span>
<span class="op">},</span>
<span class="co">// Matches any request starting with /foo</span>
<span class="st">"/foo"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"<url_2>"</span><span class="op">,</span>
<span class="st">"ssl"</span><span class="op">:</span> <span class="kw">true</span><span class="op">,</span>
<span class="st">"pathRewrite"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"^/foo"</span><span class="op">:</span> <span class="st">"/foo/beta"</span>
<span class="op">}</span>
<span class="co">// ...</span>
<span class="op">},</span>
<span class="co">// Matches /bar/abc.html but not /bar/sub/def.html</span>
<span class="st">"/bar/[^/]*[.]html"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"<url_3>"</span><span class="op">,</span>
<span class="co">// ...</span>
<span class="op">},</span>
<span class="co">// Matches /baz/abc.html and /baz/sub/def.html</span>
<span class="st">"/baz/.*/.*[.]html"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"<url_4>"</span>
<span class="co">// ...</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="co">// ...</span>
<span class="op">}</span></code></pre></div>
<h3 id="configuring-a-websocket-proxy">Configuring a WebSocket Proxy</h3>
<p>When setting up a WebSocket proxy, there are a some extra considerations to be aware of.</p>
<p>If you’re using a WebSocket engine like <a href="https://socket.io/">Socket.io</a>, you must have a Socket.io server running that you can use as the proxy target. Socket.io will not work with a standard WebSocket server. Specifically, don't expect Socket.io to work with <a href="http://websocket.org/echo.html">the websocket.org echo test</a>.</p>
<p>There’s some good documentation available for <a href="https://socket.io/docs/">setting up a Socket.io server</a>.</p>
<p>Standard WebSockets <strong>will</strong> work with a standard WebSocket server as well as the websocket.org echo test. You can use libraries like <a href="https://github.com/websockets/ws">ws</a> for the server, with <a href="https://developer.mozilla.org/en-US/docs/Web/API/WebSocket">native WebSockets in the browser</a>.</p>
<p>Either way, you can proxy WebSocket requests manually in <code>package.json</code>:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="op">{</span>
<span class="co">// ...</span>
<span class="st">"proxy"</span><span class="op">:</span> <span class="op">{</span>
<span class="st">"/socket"</span><span class="op">:</span> <span class="op">{</span>
<span class="co">// Your compatible WebSocket server</span>
<span class="st">"target"</span><span class="op">:</span> <span class="st">"ws://<socket_url>"</span><span class="op">,</span>
<span class="co">// Tell http-proxy-middleware that this is a WebSocket proxy.</span>
<span class="co">// Also allows you to proxy WebSocket requests without an additional HTTP request</span>
<span class="co">// https://github.com/chimurai/http-proxy-middleware#external-websocket-upgrade</span>
<span class="st">"ws"</span><span class="op">:</span> <span class="kw">true</span>
<span class="co">// ...</span>
<span class="op">}</span>
<span class="op">}</span>
<span class="co">// ...</span>
<span class="op">}</span></code></pre></div>
<h2 id="using-https-in-development">Using HTTPS in Development</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.4.0</code> and higher.</p>
</blockquote>
<p>You may require the dev server to serve pages over HTTPS. One particular case where this could be useful is when using <a href="#proxying-api-requests-in-development">the "proxy" feature</a> to proxy requests to an API server when that API server is itself serving HTTPS.</p>
<p>To do this, set the <code>HTTPS</code> environment variable to <code>true</code>, then start the dev server as usual with <code>npm start</code>:</p>
<h4 id="windows-cmd.exe-1">Windows (cmd.exe)</h4>
<pre class="cmd"><code>set HTTPS=true&&npm start</code></pre>
<h4 id="windows-powershell-1">Windows (Powershell)</h4>
<pre class="powershell"><code>($env:HTTPS = $true) -and (npm start)</code></pre>
<p>(Note: the lack of whitespace is intentional.)</p>
<h4 id="linux-macos-bash-1">Linux, macOS (Bash)</h4>
<div class="sourceCode"><pre class="sourceCode bash"><code class="sourceCode bash"><span class="va">HTTPS=</span>true <span class="ex">npm</span> start</code></pre></div>
<p>Note that the server will use a self-signed certificate, so your web browser will almost definitely display a warning upon accessing the page.</p>
<h2 id="generating-dynamic-meta-tags-on-the-server">Generating Dynamic <code><meta></code> Tags on the Server</h2>
<p>Since Create React App doesn’t support server rendering, you might be wondering how to make <code><meta></code> tags dynamic and reflect the current URL. To solve this, we recommend to add placeholders into the HTML, like this:</p>
<div class="sourceCode"><pre class="sourceCode html"><code class="sourceCode html"><span class="dt"><!doctype </span>html<span class="dt">></span>
<span class="kw"><html</span><span class="ot"> lang=</span><span class="st">"en"</span><span class="kw">></span>
<span class="kw"><head></span>
<span class="kw"><meta</span><span class="ot"> property=</span><span class="st">"og:title"</span><span class="ot"> content=</span><span class="st">"__OG_TITLE__"</span><span class="kw">></span>
<span class="kw"><meta</span><span class="ot"> property=</span><span class="st">"og:description"</span><span class="ot"> content=</span><span class="st">"__OG_DESCRIPTION__"</span><span class="kw">></span></code></pre></div>
<p>Then, on the server, regardless of the backend you use, you can read <code>index.html</code> into memory and replace <code>__OG_TITLE__</code>, <code>__OG_DESCRIPTION__</code>, and any other placeholders with values depending on the current URL. Just make sure to sanitize and escape the interpolated values so that they are safe to embed into HTML!</p>
<p>If you use a Node server, you can even share the route matching logic between the client and the server. However duplicating it also works fine in simple cases.</p>
<h2 id="pre-rendering-into-static-html-files">Pre-Rendering into Static HTML Files</h2>
<p>If you’re hosting your <code>build</code> with a static hosting provider you can use <a href="https://www.npmjs.com/package/react-snapshot">react-snapshot</a> or <a href="https://github.com/stereobooster/react-snap">react-snap</a> to generate HTML pages for each route, or relative link, in your application. These pages will then seamlessly become active, or “hydrated”, when the JavaScript bundle has loaded.</p>
<p>There are also opportunities to use this outside of static hosting, to take the pressure off the server when generating and caching routes.</p>
<p>The primary benefit of pre-rendering is that you get the core content of each page <em>with</em> the HTML payload—regardless of whether or not your JavaScript bundle successfully downloads. It also increases the likelihood that each route of your application will be picked up by search engines.</p>
<p>You can read more about <a href="https://medium.com/superhighfives/an-almost-static-stack-6df0a2791319">zero-configuration pre-rendering (also called snapshotting) here</a>.</p>
<h2 id="injecting-data-from-the-server-into-the-page">Injecting Data from the Server into the Page</h2>
<p>Similarly to the previous section, you can leave some placeholders in the HTML that inject global variables, for example:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="op"><!</span>doctype html<span class="op">></span>
<span class="op"><</span>html lang<span class="op">=</span><span class="st">"en"</span><span class="op">></span>
<span class="op"><</span>head<span class="op">></span>
<span class="op"><</span>script<span class="op">></span>
<span class="va">window</span>.<span class="at">SERVER_DATA</span> <span class="op">=</span> __SERVER_DATA__<span class="op">;</span>
<span class="op"><</span><span class="ss">/script></span></code></pre></div>
<p>Then, on the server, you can replace <code>__SERVER_DATA__</code> with a JSON of real data right before sending the response. The client code can then read <code>window.SERVER_DATA</code> to use it. <strong>Make sure to <a href="https://medium.com/node-security/the-most-common-xss-vulnerability-in-react-js-applications-2bdffbcc1fa0">sanitize the JSON before sending it to the client</a> as it makes your app vulnerable to XSS attacks.</strong></p>
<h2 id="running-tests">Running Tests</h2>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.3.0</code> and higher.<br> <a href="https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md#migrating-from-023-to-030">Read the migration guide to learn how to enable it in older projects!</a></p>
</blockquote>
<p>Create React App uses <a href="https://facebook.github.io/jest/">Jest</a> as its test runner. To prepare for this integration, we did a <a href="https://facebook.github.io/jest/blog/2016/09/01/jest-15.html">major revamp</a> of Jest so if you heard bad things about it years ago, give it another try.</p>
<p>Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.</p>
<p>While Jest provides browser globals such as <code>window</code> thanks to <a href="https://github.com/tmpvar/jsdom">jsdom</a>, they are only approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.</p>
<p>We recommend that you use a separate tool for browser end-to-end tests if you need them. They are beyond the scope of Create React App.</p>
<h3 id="filename-conventions">Filename Conventions</h3>
<p>Jest will look for test files with any of the following popular naming conventions:</p>
<ul>
<li>Files with <code>.js</code> suffix in <code>__tests__</code> folders.</li>
<li>Files with <code>.test.js</code> suffix.</li>
<li>Files with <code>.spec.js</code> suffix.</li>
</ul>
<p>The <code>.test.js</code> / <code>.spec.js</code> files (or the <code>__tests__</code> folders) can be located at any depth under the <code>src</code> top level folder.</p>
<p>We recommend to put the test files (or <code>__tests__</code> folders) next to the code they are testing so that relative imports appear shorter. For example, if <code>App.test.js</code> and <code>App.js</code> are in the same folder, the test just needs to <code>import App from './App'</code> instead of a long relative path. Colocation also helps find tests more quickly in larger projects.</p>
<h3 id="command-line-interface">Command Line Interface</h3>
<p>When you run <code>npm test</code>, Jest will launch in the watch mode. Every time you save a file, it will re-run the tests, just like <code>npm start</code> recompiles the code.</p>
<p>The watcher includes an interactive command-line interface with the ability to run all tests, or focus on a search pattern. It is designed this way so that you can keep it open and enjoy fast re-runs. You can learn the commands from the “Watch Usage” note that the watcher prints after every run:</p>
<div class="figure">
<img src="http://facebook.github.io/jest/img/blog/15-watch.gif" alt="Jest watch mode" />
<p class="caption">Jest watch mode</p>
</div>
<h3 id="version-control-integration">Version Control Integration</h3>
<p>By default, when you run <code>npm test</code>, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests run fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.</p>
<p>Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press <code>a</code> in the watch mode to force Jest to run all tests.</p>
<p>Jest will always run all tests on a <a href="#continuous-integration">continuous integration</a> server or if the project is not inside a Git or Mercurial repository.</p>
<h3 id="writing-tests">Writing Tests</h3>
<p>To create tests, add <code>it()</code> (or <code>test()</code>) blocks with the name of the test and its code. You may optionally wrap them in <code>describe()</code> blocks for logical grouping but this is neither required nor recommended.</p>
<p>Jest provides a built-in <code>expect()</code> global function for making assertions. A basic test could look like this:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> sum <span class="im">from</span> <span class="st">'./sum'</span><span class="op">;</span>
<span class="at">it</span>(<span class="st">'sums numbers'</span><span class="op">,</span> () <span class="op">=></span> <span class="op">{</span>
<span class="at">expect</span>(<span class="at">sum</span>(<span class="dv">1</span><span class="op">,</span> <span class="dv">2</span>)).<span class="at">toEqual</span>(<span class="dv">3</span>)<span class="op">;</span>
<span class="at">expect</span>(<span class="at">sum</span>(<span class="dv">2</span><span class="op">,</span> <span class="dv">2</span>)).<span class="at">toEqual</span>(<span class="dv">4</span>)<span class="op">;</span>
<span class="op">}</span>)<span class="op">;</span></code></pre></div>
<p>All <code>expect()</code> matchers supported by Jest are <a href="https://facebook.github.io/jest/docs/en/expect.html#content">extensively documented here</a>.<br> You can also use <a href="https://facebook.github.io/jest/docs/en/expect.html#tohavebeencalled"><code>jest.fn()</code> and <code>expect(fn).toBeCalled()</code></a> to create “spies” or mock functions.</p>
<h3 id="testing-components">Testing Components</h3>
<p>There is a broad spectrum of component testing techniques. They range from a “smoke test” verifying that a component renders without throwing, to shallow rendering and testing some of the output, to full rendering and testing component lifecycle and state changes.</p>
<p>Different projects choose different testing tradeoffs based on how often components change, and how much logic they contain. If you haven’t decided on a testing strategy yet, we recommend that you start with creating simple smoke tests for your components:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> ReactDOM <span class="im">from</span> <span class="st">'react-dom'</span><span class="op">;</span>
<span class="im">import</span> App <span class="im">from</span> <span class="st">'./App'</span><span class="op">;</span>
<span class="at">it</span>(<span class="st">'renders without crashing'</span><span class="op">,</span> () <span class="op">=></span> <span class="op">{</span>
<span class="kw">const</span> div <span class="op">=</span> <span class="va">document</span>.<span class="at">createElement</span>(<span class="st">'div'</span>)<span class="op">;</span>
<span class="va">ReactDOM</span>.<span class="at">render</span>(<span class="op"><</span>App /<span class="op">>,</span> div)<span class="op">;</span>
<span class="op">}</span>)<span class="op">;</span></code></pre></div>
<p>This test mounts a component and makes sure that it didn’t throw during rendering. Tests like this provide a lot of value with very little effort so they are great as a starting point, and this is the test you will find in <code>src/App.test.js</code>.</p>
<p>When you encounter bugs caused by changing components, you will gain a deeper insight into which parts of them are worth testing in your application. This might be a good time to introduce more specific tests asserting specific expected output or behavior.</p>
<p>If you’d like to test components in isolation from the child components they render, we recommend using <a href="http://airbnb.io/enzyme/docs/api/shallow.html"><code>shallow()</code> rendering API</a> from <a href="http://airbnb.io/enzyme/">Enzyme</a>. To install it, run:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save enzyme enzyme-adapter-react-16 react-test-renderer</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add enzyme enzyme-adapter-react-16 react-test-renderer</code></pre></div>
<p>As of Enzyme 3, you will need to install Enzyme along with an Adapter corresponding to the version of React you are using. (The examples above use the adapter for React 16.)</p>
<p>The adapter will also need to be configured in your <a href="#initializing-test-environment">global setup file</a>:</p>
<h4 id="srcsetuptests.js"><code>src/setupTests.js</code></h4>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> <span class="op">{</span> configure <span class="op">}</span> <span class="im">from</span> <span class="st">'enzyme'</span><span class="op">;</span>
<span class="im">import</span> Adapter <span class="im">from</span> <span class="st">'enzyme-adapter-react-16'</span><span class="op">;</span>
<span class="at">configure</span>(<span class="op">{</span> <span class="dt">adapter</span><span class="op">:</span> <span class="kw">new</span> <span class="at">Adapter</span>() <span class="op">}</span>)<span class="op">;</span></code></pre></div>
<blockquote>
<p>Note: Keep in mind that if you decide to "eject" before creating <code>src/setupTests.js</code>, the resulting <code>package.json</code> file won't contain any reference to it. <a href="#initializing-test-environment">Read here</a> to learn how to add this after ejecting.</p>
</blockquote>
<p>Now you can write a smoke test with it:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> <span class="op">{</span> shallow <span class="op">}</span> <span class="im">from</span> <span class="st">'enzyme'</span><span class="op">;</span>
<span class="im">import</span> App <span class="im">from</span> <span class="st">'./App'</span><span class="op">;</span>
<span class="at">it</span>(<span class="st">'renders without crashing'</span><span class="op">,</span> () <span class="op">=></span> <span class="op">{</span>
<span class="at">shallow</span>(<span class="op"><</span>App /<span class="op">></span>)<span class="op">;</span>
<span class="op">}</span>)<span class="op">;</span></code></pre></div>
<p>Unlike the previous smoke test using <code>ReactDOM.render()</code>, this test only renders <code><App></code> and doesn’t go deeper. For example, even if <code><App></code> itself renders a <code><Button></code> that throws, this test will pass. Shallow rendering is great for isolated unit tests, but you may still want to create some full rendering tests to ensure the components integrate correctly. Enzyme supports <a href="http://airbnb.io/enzyme/docs/api/mount.html">full rendering with <code>mount()</code></a>, and you can also use it for testing state changes and component lifecycle.</p>
<p>You can read the <a href="http://airbnb.io/enzyme/">Enzyme documentation</a> for more testing techniques. Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-in <code>expect()</code> and <code>jest.fn()</code> for spies.</p>
<p>Here is an example from Enzyme documentation that asserts specific output, rewritten to use Jest matchers:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> React <span class="im">from</span> <span class="st">'react'</span><span class="op">;</span>
<span class="im">import</span> <span class="op">{</span> shallow <span class="op">}</span> <span class="im">from</span> <span class="st">'enzyme'</span><span class="op">;</span>
<span class="im">import</span> App <span class="im">from</span> <span class="st">'./App'</span><span class="op">;</span>
<span class="at">it</span>(<span class="st">'renders welcome message'</span><span class="op">,</span> () <span class="op">=></span> <span class="op">{</span>
<span class="kw">const</span> wrapper <span class="op">=</span> <span class="at">shallow</span>(<span class="op"><</span>App /<span class="op">></span>)<span class="op">;</span>
<span class="kw">const</span> welcome <span class="op">=</span> <span class="op"><</span>h2<span class="op">></span>Welcome to React<span class="op"><</span><span class="ss">/h2>;</span>
<span class="ss"> // expect</span><span class="sc">(</span><span class="ss">wrapper.contains</span><span class="sc">(</span><span class="ss">welcome</span><span class="sc">))</span><span class="ss">.to.equal</span><span class="sc">(</span><span class="ss">true</span><span class="sc">)</span><span class="ss">;</span>
<span class="ss"> expect</span><span class="sc">(</span><span class="ss">wrapper.contains</span><span class="sc">(</span><span class="ss">welcome</span><span class="sc">))</span><span class="ss">.toEqual</span><span class="sc">(</span><span class="ss">true</span><span class="sc">)</span><span class="ss">;</span>
<span class="ss">}</span><span class="sc">)</span><span class="ss">;</span></code></pre></div>
<p>All Jest matchers are <a href="http://facebook.github.io/jest/docs/en/expect.html">extensively documented here</a>.<br> Nevertheless you can use a third-party assertion library like <a href="http://chaijs.com/">Chai</a> if you want to, as described below.</p>
<p>Additionally, you might find <a href="https://github.com/blainekasten/enzyme-matchers">jest-enzyme</a> helpful to simplify your tests with readable matchers. The above <code>contains</code> code can be written more simply with jest-enzyme.</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="at">expect</span>(wrapper).<span class="at">toContainReact</span>(welcome)</code></pre></div>
<p>To enable this, install <code>jest-enzyme</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">npm</span> install --save jest-enzyme</code></pre></div>
<p>Alternatively you may use <code>yarn</code>:</p>
<div class="sourceCode"><pre class="sourceCode sh"><code class="sourceCode bash"><span class="ex">yarn</span> add jest-enzyme</code></pre></div>
<p>Import it in <a href="#initializing-test-environment"><code>src/setupTests.js</code></a> to make its matchers available in every test:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> <span class="st">'jest-enzyme'</span><span class="op">;</span></code></pre></div>
<h3 id="using-third-party-assertion-libraries">Using Third Party Assertion Libraries</h3>
<p>We recommend that you use <code>expect()</code> for assertions and <code>jest.fn()</code> for spies. If you are having issues with them please <a href="https://github.com/facebook/jest/issues/new">file those against Jest</a>, and we’ll fix them. We intend to keep making them better for React, supporting, for example, <a href="https://github.com/facebook/jest/pull/1566">pretty-printing React elements as JSX</a>.</p>
<p>However, if you are used to other libraries, such as <a href="http://chaijs.com/">Chai</a> and <a href="http://sinonjs.org/">Sinon</a>, or if you have existing code using them that you’d like to port over, you can import them normally like this:</p>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="im">import</span> sinon <span class="im">from</span> <span class="st">'sinon'</span><span class="op">;</span>
<span class="im">import</span> <span class="op">{</span> expect <span class="op">}</span> <span class="im">from</span> <span class="st">'chai'</span><span class="op">;</span></code></pre></div>
<p>and then use them in your tests like you normally do.</p>
<h3 id="initializing-test-environment">Initializing Test Environment</h3>
<blockquote>
<p>Note: this feature is available with <code>react-scripts@0.4.0</code> and higher.</p>
</blockquote>
<p>If your app uses a browser API that you need to mock in your tests or if you just need a global setup before running your tests, add a <code>src/setupTests.js</code> to your project. It will be automatically executed before running your tests.</p>
<p>For example:</p>
<h4 id="srcsetuptests.js-1"><code>src/setupTests.js</code></h4>
<div class="sourceCode"><pre class="sourceCode js"><code class="sourceCode javascript"><span class="kw">const</span> localStorageMock <span class="op">=</span> <span class="op">{</span>
<span class="dt">getItem</span><span class="op">:</span> <span class="va">jest</span>.<span class="at">fn</span>()<span class="op">,</span>
<span class="dt">setItem</span><span class="op">:</span> <span class="va">jest</span>.<span class="at">fn</span>()<span class="op">,</span>
<span class="dt">clear</span><span class="op">:</span> <span class="va">jest</span>.<span class="at">fn</span>()
<span class="op">};</span>
<span class="va">global</span>.<span class="at">localStorage</span> <span class="op">=</span> localStorageMock</code></pre></div>
<blockquote>
<p>Note: Keep in mind that if you decide to "eject" before creating <code>src/setupTests.js</code>, the resulting <code>package.json</code> file won't contain any reference to it, so you should manually create the property <code>setupTestFrameworkScriptFile</code> in the configuration for Jest, something like the following:</p>
</blockquote>
<blockquote>
<p><code>js "jest": { // ... "setupTestFrameworkScriptFile": "<rootDir>/src/setupTests.js" }</code></p>
</blockquote>
<h3 id="focusing-and-excluding-tests">Focusing and Excluding Tests</h3>
<p>You can replace <code>it()</code> with <code>xit()</code> to temporarily exclude a test from being executed.<br> Similarly, <code>fit()</code> lets you focus on a specific test without running any other tests.</p>
<h3 id="coverage-reporting">Coverage Reporting</h3>
<p>Jest has an integrated coverage reporter that works well with ES6 and requires no configuration.<br> Run <code>npm test -- --coverage</code> (note extra <code>--</code> in the middle) to include a coverage report like this:</p>
<div class="figure">
<img src="http://i.imgur.com/5bFhnTS.png" alt="coverage report" />
<p class="caption">coverage report</p>
</div>
<p>Note that tests run much slower with coverage so it is recommended to run it separately from your normal workflow.</p>
<h4 id="configuration">Configuration</h4>
<p>The default Jest coverage configuration can be overriden by adding any of the following supported keys to a Jest config in your package.json.</p>
<p>Supported overrides: - <a href="https://facebook.github.io/jest/docs/en/configuration.html#collectcoveragefrom-array"><code>collectCoverageFrom</code></a> - <a href="https://facebook.github.io/jest/docs/en/configuration.html#coveragereporters-array-string"><code>coverageReporters</code></a> - <a href="https://facebook.github.io/jest/docs/en/configuration.html#coveragethreshold-object"><code>coverageThreshold</code></a> - <a href="https://facebook.github.io/jest/docs/en/configuration.html#snapshotserializers-array-string"><code>snapshotSerializers</code></a></p>
<p>Example package.json:</p>
<div class="sourceCode"><pre class="sourceCode json"><code class="sourceCode json"><span class="fu">{</span>
<span class="dt">"name"</span><span class="fu">:</span> <span class="st">"your-package"</span><span class="fu">,</span>
<span class="dt">"jest"</span><span class="fu">:</span> <span class="fu">{</span>
<span class="dt">"collectCoverageFrom"</span> <span class="fu">:</span> <span class="ot">[</span>
<span class="st">"src/**/*.{js,jsx}"</span><span class="ot">,</span>
<span class="st">"!<rootDir>/node_modules/"</span><span class="ot">,</span>
<span class="st">"!<rootDir>/path/to/dir/"</span>
<span class="ot">]</span><span class="fu">,</span>
<span class="dt">"coverageThreshold"</span><span class="fu">:</span> <span class="fu">{</span>
<span class="dt">"global"</span><span class="fu">:</span> <span class="fu">{</span>