View in EnglishAlways switch to English

Map

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨2015年7月⁩.

* Some parts of this feature may have varying levels of support.

Map 是保存了鍵值對（key-value pairs）的物件。任何值（包括物件及基本型別（primitive）值）都可以作為鍵或值。

語法

new Map([iterable])

參數

iterable: 為一個陣列或其他元素成鍵值對的可迭代物件（有兩個元素的陣列，例如[[ 1, 'one' ],[ 2, 'two' ]]）。每一個鍵值對都會被加入至新的Map；null 會被視為undefined。

描述

一個Map 物件會根據元素的新增順序走訪自身的所有元素 —for...of 迴圈會在每次迭代回傳一個[key, value] 陣列。

鍵的相等性

鍵相等是基於SameValueZero 的演算法：NaN 被認為與NaN 相同（即使NaN !== NaN）並且根據=== 運算符的語義，所有其他值都被認為相等。在目前的 ECMAScript 規範中，-0 和+0 被認為是相等的，儘管在早期的草案中並非如此。詳細的內容請參閱瀏覽器相容性表中的 "Value equality for -0 and 0"。

Object 及 Map 的比較

Object 和Map 類似。兩者都允許你設置對應的鍵值對，檢索這些值，刪除鍵，並檢測是否有什麼存儲在鍵中。正因為如此（也因為沒有內置的替代品），Object 在歷史上一直被當作Map 使用；然而在某些情況下，使用Map 有一些重要的不同之處:

Object 的鍵是字串 和Symbol，而它們在Map 中可以是任意的資料型態，包括函數，對象以及原始的資料型態。
你可以使用size 屬性輕鬆地獲得Map 的大小，而Object 中的屬性數量必須手動確認。
Map 是可迭代（iterable）的，因此可以直接迭代，而在Object 上迭代則需要以某種方式獲取其鍵並對其進行迭代。
Object 有一個原型，所以如果不小心，映射中有一些默認鍵可能與鍵發生衝突。從 ES5 開始，這可以通過使用map = Object.create(null) 來繞過這個問題，但是很少這樣做。
在涉及頻繁添加和刪除鍵值對的場景中，Map 可能表現得更好。

屬性

Map.length: length屬性的值為 0要計算Map 中有多少元素，可以使用Map.prototype.size。
Map[Symbol.species]: 用於創建派生物件的構造函數。
Map.prototype: 表示Map 構造函數的原型，允許對所有的Map 物件添加屬性

`Map` 物件實體

所有的Map 實例都繼承自Map.prototype.

屬性

Map.prototype[Symbol.toStringTag]: The initial value of theSymbol.toStringTag property is the string"Map". This property is used inObject.prototype.toString().
Map.prototype.size: Returns the number of key/value pairs in theMap object.

方法

Map.prototype.clear(): Removes all key-value pairs from theMap object.
Map.prototype.delete(): Returnstrue if an element in theMap object existed and has beenremoved, orfalse if the element does not exist.map.has(key)will returnfalse afterwards.
Map.prototype.get(): Returns the value associated to the passed key, orundefined if there is none.
Map.prototype.has(): Returns a boolean indicating whether a value has been associated with the passed key in theMap object or not.
Map.prototype.set(): Sets the value for the passed key in theMap object. Returns theMap object.
Map.prototype[Symbol.iterator](): Returns a new Iterator object that contains a two-member array of[key, value] for each element in theMap object in insertion order.
Map.prototype.keys(): Returns a new Iterator object that contains the keys for each element in theMap object in insertion order.
Map.prototype.values(): Returns a new Iterator object that contains the values for each element in theMap object in insertion order.
Map.prototype.entries(): Returns a new Iterator object that contains a two-member array of[key, value] for each element in theMap object in insertion order.
Map.prototype.forEach(): CallscallbackFn once for each key-value pair present in theMap object, in insertion order. If athisArg parameter is provided toforEach, it will be used as thethis value for each callback.

範例

使用`Map` 物件

var myMap = new Map();var keyString = "a string",  keyObj = {},  keyFunc = function () {};// setting the valuesmyMap.set(keyString, "value associated with 'a string'");myMap.set(keyObj, "value associated with keyObj");myMap.set(keyFunc, "value associated with keyFunc");myMap.size; // 3// getting the valuesmyMap.get(keyString); // "value associated with 'a string'"myMap.get(keyObj); // "value associated with keyObj"myMap.get(keyFunc); // "value associated with keyFunc"myMap.get("a string"); // "value associated with 'a string'"// because keyString === 'a string'myMap.get({}); // undefined, because keyObj !== {}myMap.get(function () {}); // undefined, because keyFunc !== function () {}

使用`NaN` 作為`Map` 的鍵

NaN 同樣可以作為Map 的 key，雖然每個NaN 都不等於自己本身，下面的例子是有效的，因為NaN 無法區分彼此。

var myMap = new Map();myMap.set(NaN, "not a number");myMap.get(NaN); // "not a number"var otherNaN = Number("foo");myMap.get(otherNaN); // "not a number"

透過`for..of` 迭代`Map`

Map 可以使用for..of 迴圈進行迭代：

var myMap = new Map();myMap.set(0, "zero");myMap.set(1, "one");for (var [key, value] of myMap) {  console.log(key + " = " + value);}// 0 = zero// 1 = onefor (var key of myMap.keys()) {  console.log(key);}// 0// 1for (var value of myMap.values()) {  console.log(value);}// zero// onefor (var [key, value] of myMap.entries()) {  console.log(key + " = " + value);}// 0 = zero// 1 = one

透過`forEach()` 迭代`Map`

Map 可以使用forEach() 方法進行迭代：

myMap.forEach(function (value, key) {  console.log(key + " = " + value);});// Will show 2 logs; first with "0 = zero" and second with "1 = one"

與`Array` 物件關聯

var kvArray = [  ["key1", "value1"],  ["key2", "value2"],];// Use the regular Map constructor to transform a 2D key-value Array into a mapvar myMap = new Map(kvArray);myMap.get("key1"); // returns "value1"// Use the Array.from function to transform a map into a 2D key-value Arrayconsole.log(Array.from(myMap)); // Will show you exactly the same Array as kvArray// Or use the keys or values iterators and convert them to an arrayconsole.log(Array.from(myMap.keys())); // Will show ["key1", "key2"]

複製與合併`Map`

就像Array 一樣，Map 可以被複製:

var original = new Map([[1, "one"]]);var clone = new Map(original);console.log(clone.get(1)); // oneconsole.log(original === clone); // false. Useful for shallow comparison

請記住，數據本身並非克隆的。

Map 可以被合併，保持鍵的唯一性:

var first = new Map([  [1, "one"],  [2, "two"],  [3, "three"],]);var second = new Map([  [1, "uno"],  [2, "dos"],]);// Merge two maps. The last repeated key wins.// Spread operator essentially converts a Map to an Arrayvar merged = new Map([...first, ...second]);console.log(merged.get(1)); // unoconsole.log(merged.get(2)); // dosconsole.log(merged.get(3)); // three

Map 也可以跟Array 合併：

var first = new Map([  [1, "one"],  [2, "two"],  [3, "three"],]);var second = new Map([  [1, "uno"],  [2, "dos"],]);// Merge maps with an array. The last repeated key wins.var merged = new Map([...first, ...second, [1, "eins"]]);console.log(merged.get(1)); // einsconsole.log(merged.get(2)); // dosconsole.log(merged.get(3)); // three

規格

Specification
ECMAScript® 2026 Language Specification # sec-map-objects

瀏覽器相容性

參見

Help improve MDN

Learn how to contribute

This page was last modified on⁨2025年7月14日⁩ byMDN contributors.

View this page on GitHub •Report a problem with this content

Movatterモバイル変換

Map

Help improve MDN