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
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 the
Symbol.toStringTagproperty is the string"Map". This property is used inObject.prototype.toString(). Map.prototype.size-
Returns the number of key/value pairs in the
Mapobject.
方法
Map.prototype.clear()-
Removes all key-value pairs from the
Mapobject. Map.prototype.delete()-
Returns
trueif an element in theMapobject existed and has been removed, orfalseif the element does not exist.map.has(key)will returnfalseafterwards. Map.prototype.get()-
Returns the value associated to the passed key, or
undefinedif there is none. Map.prototype.has()-
Returns a boolean indicating whether a value has been associated with the passed key in the
Mapobject or not. Map.prototype.set()-
Sets the value for the passed key in the
Mapobject. Returns theMapobject. Map.prototype[Symbol.iterator]()-
Returns a new Iterator object that contains a two-member array of
[key, value]for each element in theMapobject in insertion order. Map.prototype.keys()-
Returns a new Iterator object that contains the keys for each element in the
Mapobject in insertion order. Map.prototype.values()-
Returns a new Iterator object that contains the values for each element in the
Mapobject in insertion order. Map.prototype.entries()-
Returns a new Iterator object that contains a two-member array of
[key, value]for each element in theMapobject in insertion order. Map.prototype.forEach()-
Calls
callbackFnonce for each key-value pair present in theMapobject, in insertion order. If athisArgparameter is provided toforEach, it will be used as thethisvalue for each callback.
範例
使用 Map 物件
var myMap = new Map(); var keyString = "a string", keyObj = {}, keyFunc = function () {}; // setting the values myMap.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 values myMap.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 = one for (var key of myMap.keys()) { console.log(key); } // 0 // 1 for (var value of myMap.values()) { console.log(value); } // zero // one for (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 map var myMap = new Map(kvArray); myMap.get("key1"); // returns "value1" // Use the Array.from function to transform a map into a 2D key-value Array console.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 array console.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)); // one console.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 Array var merged = new Map([...first, ...second]); console.log(merged.get(1)); // uno console.log(merged.get(2)); // dos console.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)); // eins console.log(merged.get(2)); // dos console.log(merged.get(3)); // three 規格
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-map-objects |