Array

elementN

除了只傳遞一個參數給 Array 構造函數，且該參數為一個數字的情況（詳見下方的 arrayLength 參數），JavaScript 陣列會以傳入的元素進行初始化。請注意，這種特殊情況僅適用於以 Array 構造函數建立的 JavaScript 陣列，而不適用於以括號語法建立的陣列常值（Array Literals）。

arrayLength

如果傳遞給 Array 構造函數的唯一參數是 0 和 2^32 - 1（含）之間的整數，將回傳一個新的 JavaScript 陣列，其長度被設定為這個數字。如果參數是任何其他數值，將拋出 RangeError 異常。

Array（「陣列」）是類似列表（list）的物件（Object），它們的原型（Prototype）擁有方法（methods）來執行遍歷和變異操作。JavaScript 陣列的長度（元素數量），以及其元素的類型都不是固定的。取決於工程師如何選擇使用陣列，可以隨時更改陣列的長度，也可不連續儲存資料， 所以並不保證這些資料是集中的。一般情況下，這些特性很方便使用；但若這些功能都不符合你的用途，你可能會想使用型別陣列（typed arrays）。

有些人認為即便會發生警告，仍然不應該使用關聯陣列，而應該使用 objects。你可參考輕量級 JavaScript 字典當中的範例。

JavaScript 陣列是 zero-indexed：陣列元素的索引值編排從 0 開始，而最後一個元素的索引值等同於陣列的 length 屬性減 1。

var arr = ["this is the first element", "this is the second element"];
console.log(arr[0]); // 紀錄出 'this is the first element'
console.log(arr[1]); // 記錄出 'this is the second element'
console.log(arr[arr.length - 1]); // 記錄出 'this is the second element'

Array 元素同時也是物件的屬性，與 toString 是一種屬性相同。但若要透過下面這種方式存取陣列元素，因為屬性名稱無效的關係，會發生語法錯誤：

console.log(arr.0); // 語法錯誤

會造成如此的原因沒有什麼特別的，在 JavaScript 當中無法用小數點的方式來參照一個名稱開頭為數字的屬性，而必須括號的表示方式來存取。舉例來說，若你有個物件的屬性名稱為「3d」，就只能用括號的方式來參照。

請看下列範例：

var years = [1950, 1960, 1970, 1980, 1990, 2000, 2010];
console.log(years.0);   // 語法錯誤
console.log(years[0]);  // 程式正常

renderer.3d.setTexture(model, 'character.png');     // 語法錯誤
renderer['3d'].setTexture(model, 'character.png');  // 程式正常

注意：以這個 '3d' 例子來說，必須用引號將 3d 包起來。你也可以將 JavaScript 陣列的索引用引號包起來（例如使用 years['2'] 而不用 years[2]），但這不是必要的。JavaScript 會透過隱含的 toString，將 years[2] 當中的 2 強制轉換為字串。由於這個原因，'2' 與 '02' 會參照到 years 物件中的不同項目，下列程式範例結果可能回傳 true：

console.log(years["2"] != years["02"]);

另一種類似的情況是，物件屬性剛好與保留字（！）相同的情況。這種情況下僅能透過括號表示方式當中的字串常值來存取：

var promise = {
  var: "text",
  array: [1, 2, 3, 4],
};

console.log(promise["var"]);

JavaScript 陣列的 length 屬性和其數值屬性相關。許多陣列的方法被呼叫時會參考 length 屬性的值（例如 join、slice、indexOf 等）。而有另一些方法則會去改變 length 屬性的值，如 push、splice。

var fruits = [];
fruits.push("banana", "apple", "peach");

console.log(fruits.length); // 3

如果給陣列設定一個數值屬性，其值為有效但超過當下範圍的陣列 index，JavaScript 引擎會依照此數值更新陣列的 length 屬性：

fruits[5] = "mango";
console.log(fruits[5]); // 'mango'
console.log(Object.keys(fruits)); // ['0', '1', '2', '5']
console.log(fruits.length); // 6

提高 length 屬性。

fruits.length = 10;
console.log(Object.keys(fruits)); // ['0', '1', '2', '5']
console.log(fruits.length); // 10

降低 length 屬性則會刪除陣列元素。

fruits.length = 2;
console.log(Object.keys(fruits)); // ['0', '1']
console.log(fruits.length); // 2

在 Array.length 頁面裡有進一步解釋。

在字串與正規表示式之間的比對結果會產生一個 javascript 陣列。此陣列內含關於比對資訊的屬性與元素。 這樣的陣列由RegExp.exec, String.match, 和 String.replace 所產生。參考以下範例和表格，會有助於說明這些屬性和元素：

// 比對一個字元 d，後面接著一或多個 b，再接著一個 d
// Remember matched b's and the following d
// 忽略大小寫

var myRe = /d(b+)(d)/i;
var myArray = myRe.exec("cdbBdbsbz");

這項比對結果的屬性與元素參考如下：

Array.length

Array 建構子的長度為 1。

Array[Symbol.species]

用來建立衍生物件的建構函數。

Array.prototype

可加入屬性至所有陣列物件。

Array.from()

用類似陣列或可列舉物件，來建立新的 Array 實例。

Array.isArray()

若變數是陣列就回傳 true，否則回傳 false。

Array.of()

用可變數量的引數來建立新的 Array 實例，不論引數的數量或型別。

所有的陣列實例都繼承自 Array.prototype。若修改這個陣列建構子 (Array constructor) 的原型物件 (prototype object)，將會影響所有的陣列實體。

Array.prototype.length

Reflects the number of elements in an array.

Array.prototype[Symbol.unscopables]

Contains property names that were not included in the ECMAScript standard prior to the ES2015 version and that are ignored for with statement-binding purposes.

Array.prototype.at()

Returns the array item at the given index. Accepts negative integers, which count back from the last item.

Array.prototype.concat()

Returns a new array that is the calling array joined with other array(s) and/or value(s).

Array.prototype.copyWithin()

Copies a sequence of array elements within an array.

Array.prototype.entries()

Returns a new array iterator object that contains the key/value pairs for each index in an array.

Array.prototype.every()

Returns true if every element in the calling array satisfies the testing function.

Array.prototype.fill()

Fills all the elements of an array from a start index to an end index with a static value.

Array.prototype.filter()

Returns a new array containing all elements of the calling array for which the provided filtering function returns true.

Array.prototype.find()

Returns the value of the first element in the array that satisfies the provided testing function, or undefined if no appropriate element is found.

Array.prototype.findIndex()

Returns the index of the first element in the array that satisfies the provided testing function, or -1 if no appropriate element was found.

Array.prototype.findLast()

Returns the value of the last element in the array that satisfies the provided testing function, or undefined if no appropriate element is found.

Array.prototype.findLastIndex()

Returns the index of the last element in the array that satisfies the provided testing function, or -1 if no appropriate element was found.

Array.prototype.flat()

Returns a new array with all sub-array elements concatenated into it recursively up to the specified depth.

Array.prototype.flatMap()

Returns a new array formed by applying a given callback function to each element of the calling array, and then flattening the result by one level.

Array.prototype.forEach()

Calls a function for each element in the calling array.

Array.prototype.group() 實驗性質

Groups the elements of an array into an object according to the strings returned by a test function.

Array.prototype.groupToMap() 實驗性質

Groups the elements of an array into a Map according to values returned by a test function.

Array.prototype.includes()

Determines whether the calling array contains a value, returning true or false as appropriate.

Array.prototype.indexOf()

Returns the first (least) index at which a given element can be found in the calling array.

Array.prototype.join()

Joins all elements of an array into a string.

Array.prototype.keys()

Returns a new array iterator that contains the keys for each index in the calling array.

Array.prototype.lastIndexOf()

Returns the last (greatest) index at which a given element can be found in the calling array, or -1 if none is found.

Array.prototype.map()

Returns a new array containing the results of invoking a function on every element in the calling array.

Array.prototype.pop()

Removes the last element from an array and returns that element.

Array.prototype.push()

Adds one or more elements to the end of an array, and returns the new length of the array.

Array.prototype.reduce()

Executes a user-supplied "reducer" callback function on each element of the array (from left to right), to reduce it to a single value.

Array.prototype.reduceRight()

Executes a user-supplied "reducer" callback function on each element of the array (from right to left), to reduce it to a single value.

Array.prototype.reverse()

Reverses the order of the elements of an array in place. (First becomes the last, last becomes first.)

Array.prototype.shift()

Removes the first element from an array and returns that element.

Array.prototype.slice()

Extracts a section of the calling array and returns a new array.

Array.prototype.some()

Returns true if at least one element in the calling array satisfies the provided testing function.

Array.prototype.sort()

Sorts the elements of an array in place and returns the array.

Array.prototype.splice()

Adds and/or removes elements from an array.

Array.prototype.toLocaleString()

Returns a localized string representing the calling array and its elements. Overrides the Object.prototype.toLocaleString() method.

Array.prototype.toString()

Returns a string representing the calling array and its elements. Overrides the Object.prototype.toString() method.

Array.prototype.unshift()

Adds one or more elements to the front of an array, and returns the new length of the array.

Array.prototype.values()

Returns a new array iterator object that contains the values for each index in the array.

Array.prototype[Symbol.iterator]()

An alias for the values() method by default.

以下範例會產生長度為 0 的 msgArray 陣列，然後指派字串值到 msgArray[0] 及 msgArray[99]，使陣列的長度變為 100。

var msgArray = [];
msgArray[0] = "Hello";
msgArray[99] = "world";

if (msgArray.length === 100) {
  console.log("The length is 100.");
}

以下範例會用字串產生一張西洋棋盤的二維陣列。第一步是將士兵 'p' 從 (6,4) 移動至 (4,4)，然後清空原本的位置 (6,4)。

var board = [
  ["R", "N", "B", "Q", "K", "B", "N", "R"],
  ["P", "P", "P", "P", "P", "P", "P", "P"],
  [" ", " ", " ", " ", " ", " ", " ", " "],
  [" ", " ", " ", " ", " ", " ", " ", " "],
  [" ", " ", " ", " ", " ", " ", " ", " "],
  [" ", " ", " ", " ", " ", " ", " ", " "],
  ["p", "p", "p", "p", "p", "p", "p", "p"],
  ["r", "n", "b", "q", "k", "b", "n", "r"],
];

console.log(board.join("\n") + "\n\n");

// 將士兵往前移兩步
board[4][4] = board[6][4];
board[6][4] = " ";
console.log(board.join("\n"));

以下是輸出結果：

R,N,B,Q,K,B,N,R
P,P,P,P,P,P,P,P
 , , , , , , ,
 , , , , , , ,
 , , , , , , ,
 , , , , , , ,
p,p,p,p,p,p,p,p
r,n,b,q,k,b,n,r

R,N,B,Q,K,B,N,R
P,P,P,P,P,P,P,P
 , , , , , , ,
 , , , , , , ,
 , , , ,p, , ,
 , , , , , , ,
p,p,p,p, ,p,p,p
r,n,b,q,k,b,n,r

values = [];
for (var x = 0; x < 10; x++) {
  values.push([2 ** x, 2 * x ** 2]);
}
console.table(values);

結果會是

0 1   0
1 2   2
2 4   8
3 8   18
4 16  32
5 32  50
6 64  72
7 128 98
8 256 128
9 512 162

（第一欄為索引）

• JavaScript Guide: 「Indexing object properties」
• JavaScript Guide: 「Predefined Core Objects: Array Object」
• Array comprehensions
• Polyfill for JavaScript 1.8.5 Array Generics and ECMAScript 5 Array Extras
• Typed Arrays