use toStringTag as type brand

Author: yaacovCR

src/jsutils/__tests__/instanceOf-test.ts

@@ -6,15 +6,14 @@ import { instanceOf } from '../instanceOf.js';
 describe('instanceOf', () => {
   it('do not throw on values without prototype', () => {
     class Foo {
-      readonly __isFoo = true as const;
       get [Symbol.toStringTag]() {
         return 'Foo';
       }
     }
 
-    expect(instanceOf(undefined, true, Foo)).to.equal(false);
-    expect(instanceOf(undefined, null, Foo)).to.equal(false);
-    expect(instanceOf(undefined, Object.create(null), Foo)).to.equal(false);
+    expect(instanceOf(false, true, Foo)).to.equal(false);
+    expect(instanceOf(false, null, Foo)).to.equal(false);
+    expect(instanceOf(false, Object.create(null), Foo)).to.equal(false);
   });
 
   it('detect name clashes with older versions of this lib', () => {
@@ -25,7 +24,6 @@ describe('instanceOf', () => {
 
     function newVersion() {
       class Foo {
-        readonly __isFoo = true as const;
         get [Symbol.toStringTag]() {
           return 'Foo';
         }
@@ -36,16 +34,13 @@ describe('instanceOf', () => {
     const NewClass = newVersion();
     const OldClass = oldVersion();
     const newInstance = new NewClass();
-    expect(instanceOf(newInstance.__isFoo, newInstance, NewClass)).to.equal(
-      true,
-    );
-    expect(() => instanceOf(undefined, new OldClass(), NewClass)).to.throw();
+    expect(instanceOf(true, newInstance, NewClass)).to.equal(true);
+    expect(() => instanceOf(false, new OldClass(), NewClass)).to.throw();
   });
 
   it('allows instances to have share the same constructor name', () => {
     function getMinifiedClass(tag: string) {
       class SomeNameAfterMinification {
-        readonly [tag] = true as const;
         get [Symbol.toStringTag]() {
           return tag;
         }
@@ -57,23 +52,17 @@ describe('instanceOf', () => {
     const Bar = getMinifiedClass('Bar');
     const fooInstance = new Foo();
     const barInstance = new Bar();
-    expect(instanceOf(fooInstance.foo, fooInstance, Bar)).to.equal(false);
-    expect(instanceOf(barInstance.bar, barInstance, Foo)).to.equal(false);
+    expect(instanceOf(false, fooInstance, Bar)).to.equal(false);
+    expect(instanceOf(false, barInstance, Foo)).to.equal(false);
 
     const DuplicateOfFoo = getMinifiedClass('Foo');
-    const duplicateOfFooInstance = new DuplicateOfFoo();
-    expect(() =>
-      instanceOf(duplicateOfFooInstance.foo, new DuplicateOfFoo(), Foo),
-    ).to.throw();
-    expect(() =>
-      instanceOf(fooInstance.foo, fooInstance, DuplicateOfFoo),
-    ).to.throw();
+    expect(() => instanceOf(true, new DuplicateOfFoo(), Foo)).to.throw();
+    expect(() => instanceOf(true, fooInstance, DuplicateOfFoo)).to.throw();
   });
 
   it('fails with descriptive error message', () => {
     function getFoo() {
       class Foo {
-        readonly __isFoo = true as const;
         get [Symbol.toStringTag]() {
           return 'Foo';
         }
@@ -86,11 +75,11 @@ describe('instanceOf', () => {
     const foo1Instance = new Foo1();
     const foo2Instance = new Foo2();
 
-    expect(() => instanceOf(foo1Instance.__isFoo, foo1Instance, Foo2)).to.throw(
-      /^Cannot use Foo "{ __isFoo: true }" from another module or realm./m,
+    expect(() => instanceOf(true, foo1Instance, Foo2)).to.throw(
+      /^Cannot use Foo "{}" from another module or realm./m,
     );
-    expect(() => instanceOf(foo2Instance.__isFoo, foo2Instance, Foo1)).to.throw(
-      /^Cannot use Foo "{ __isFoo: true }" from another module or realm./m,
+    expect(() => instanceOf(true, foo2Instance, Foo1)).to.throw(
+      /^Cannot use Foo "{}" from another module or realm./m,
     );
   });
 });

src/jsutils/instanceOf.ts

@@ -16,18 +16,18 @@ const isProduction =
  * See: https://webpack.js.org/guides/production/
  */
 export const instanceOf: (
-  typeBrand: true | undefined,
+  isBranded: boolean,
   value: unknown,
   constructor: Constructor,
 ) => boolean =
   /* c8 ignore next 9 */
   // FIXME: https://github.com/graphql/graphql-js/issues/2317
   isProduction
-    ? function instanceOf(typeBrand: true | undefined): boolean {
-        return typeBrand === true;
+    ? function instanceOf(isBranded: boolean): boolean {
+        return isBranded;
       }
     : function instanceOf(
-        _typeBrand: true | undefined,
+        _isBranded: boolean,
         value: unknown,
         constructor: Constructor,
       ): boolean {

src/language/source.ts

@@ -14,8 +14,6 @@ interface Location {
  * The `line` and `column` properties in `locationOffset` are 1-indexed.
  */
 export class Source {
-  readonly __isSource = true as const;
-
   body: string;
   name: string;
   locationOffset: Location;
@@ -49,5 +47,9 @@ export class Source {
  * @internal
  */
 export function isSource(source: unknown): source is Source {
-  return instanceOf((source as any)?.__isSource, source, Source);
+  return instanceOf(
+    (source as any)?.[Symbol.toStringTag] === 'Source',
+    source,
+    Source,
+  );
 }