javascript-data.md

title
JavaScript / Data Structure

JavaScript / Data Structure

Array

• Array literals - Grammar and types - JavaScript | MDN

  • An array literal is a list of zero or more expressions, each of which represents an array element, enclosed in square brackets ([]). When you create an array using an array literal, it is initialized with the specified values as its elements, and its length is set to the number of arguments specified.

  • The following example creates the coffees array with three elements and a length of three:

    var coffees = ['French Roast', 'Colombian', 'Kona'];
    

    Note : An array literal is a type of OBJECT INITIALIZER. See Using Object Initializers.

  • If an array is created using a literal in a TOP-LEVEL SCRIPT, JavaScript interprets the array each time it evaluates the expression containing the array literal. In addition, a literal used in a function is CREATED EACH TIME the function is called.

    什麼時候不會遇到一次就 evaluate 一次 ??

  • Array literals are also Array objects. See Array and Indexed collections for details on Array objects.

    另外還有 array-like objects。

  Extra commas in array literals

  • You do not have to specify all elements in an array literal. If you put two commas in a row, the array is created with undefined for the unspecified elements. The following example creates the fish array:

    var fish = ['Lion', , 'Angel'];
    

    This array has two elements with values and one EMPTY ELEMENT (fish[0] is "Lion", fish[1] is undefined, and fish[2] is "Angel").

  • If you include a trailing comma at the end of the list of elements, the comma is IGNORED. In the following example, the length of the array is three. There is no myList[3]. All other commas in the list indicate a new element.

    Note : Trailing commas can create errors in older browser versions and it is a BEST PRACTICE TO REMOVE THEM.

    var myList = ['home', , 'school', ];
    

  • In the following example, the length of the array is four, and myList[0] and myList[2] are missing.

    var myList = [ ,'home', , 'school'];
    

  • In the following example, the length of the array is four, and myList[1] and myList[3] are missing. Only the last comma is ignored.

    var myList = ['home', , 'school', , ];
    

  • Understanding the behavior of extra commas is important to understanding JavaScript as a language. However, when writing your own code, explicitly declare the missing elements as undefined, as doing this increases your code's CLARITY and MAINTAINABILITY.

• Indexed collections - JavaScript | MDN #ril

• Array | MDN #ril

  • The JavaScript Array object is a global object?? that is used in the construction of arrays; which are high-level, LIST-LIKE objects.

  • Create an Array

    var fruits = ['Apple', 'Banana']; // 字面表示跟 Python 一樣
    
    console.log(fruits.length); // 長度透過 .length 取出，跟 Java array 一樣
    // 2
    

  • Access (index into) an Array item

    var first = fruits[0];
    // Apple
    
    var last = fruits[fruits.length - 1];
    // Banana
    

  • Loop over an Array

    fruits.forEach(function(item, index, array) {
      console.log(item, index);
    });
    // Apple 0
    // Banana 1
    

  • Add to the end of an Array

    var newLength = fruits.push('Orange'); // 類 stack 的操作
    // ["Apple", "Banana", "Orange"]
    

  • Remove from the end of an Array

    var last = fruits.pop(); // remove Orange (from the end)
    // ["Apple", "Banana"];
    

• Indexed collections | MDN #ril

Map

• Map | MDN #ril
• WeakMap | MDN #ril

Set

• Set | MDN #ril
• WeakSet | MDN #ril

參考資料 {: #reference }

手冊：

• Array | MDN