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 Mapnull 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browser compatibility 表中的"Value equality for -0 and 0"。

物件及 maps 的比較

Objects 和Maps 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),Objects在歷史上一直被當作Maps使用;然而在某些情況下,使用Maps有一些重要的不同之處:

  • Object 的鍵是{jsxref("String", "Strings")}} 和Symbols, 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
get Map[@@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.constructor
Returns the function that created an instance's prototype. This is the Map function by default.
Map.prototype.size
Returns the number of key/value pairs in the Map object.

方法

Map.prototype.clear()
Removes all key/value pairs from the Map object.
Map.prototype.delete(key)
Returns true if an element in the Map object existed and has been removed, or false if the element does not exist. Map.prototype.has(key) will return false afterwards.
Map.prototype.entries()
Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.
Map.prototype.forEach(callbackFn[, thisArg])
Calls callbackFn once for each key-value pair present in the Map object, in insertion order. If a thisArg parameter is provided to forEach, it will be used as the this value for each callback.
Map.prototype.get(key)
Returns the value associated to the key, or undefined if there is none.
Map.prototype.has(key)
Returns a boolean asserting whether a value has been associated to the key in the Map object or not.
Map.prototype.keys()
Returns a new Iterator object that contains the keys for each element in the Map object in insertion order.
Map.prototype.set(key, value)
Sets the value for the key in the Map object. Returns the Map object.
Map.prototype.values()
Returns a new Iterator object that contains the values for each element in the Map object in insertion order.
Map.prototype[@@iterator]()
Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.

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NaNs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 Maps

Maps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 Maps

Maps可以使用forEach()方法forEach()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 Maps

就像Arrays一樣,Maps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。

Maps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

Maps也可以跟 Arrays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

規範

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Map' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of 'Map' in that specification.
Draft  

瀏覽器相容性

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
MapChrome Full support 38Edge Full support 12Firefox Full support 13IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
Full support 0.12
Full support 0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
new Map(iterable)Chrome Full support 38Edge Full support 12Firefox Full support 13IE No support NoOpera Full support 25Safari Full support 9WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 9Samsung Internet Android Full support Yesnodejs Full support 0.12
new Map(null)Chrome Full support YesEdge Full support 12Firefox Full support 37IE Full support 11Opera Full support YesSafari Full support 9WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support 12Firefox Android Full support 37Opera Android Full support YesSafari iOS Full support 9Samsung Internet Android Full support Yesnodejs Full support 0.12
Full support 0.12
Full support 0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
Map() without new throwsChrome Full support YesEdge Full support 12Firefox Full support 42IE Full support 11Opera Full support YesSafari Full support 9WebView Android Full support YesChrome Android Full support YesEdge Mobile Full support 12Firefox Android Full support 42Opera Android Full support YesSafari iOS Full support 9Samsung Internet Android Full support Yesnodejs Full support 0.12
Key equality for -0 and 0Chrome Full support 38Edge Full support 12Firefox Full support 29IE No support NoOpera Full support 25Safari Full support 9WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 29Opera Android Full support 25Safari iOS Full support 9Samsung Internet Android Full support Yesnodejs Full support 4.0.0
clearChrome Full support 38Edge Full support 12Firefox Full support 19IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 19Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
deleteChrome Full support 38Edge Full support 12Firefox Full support 13IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
Full support 0.12
Full support 0.10
Disabled
Disabled From version 0.10: this feature is behind the --harmony runtime flag.
entriesChrome Full support 38Edge Full support 12Firefox Full support 20IE No support NoOpera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 20Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
forEachChrome Full support 38Edge Full support 12Firefox Full support 25IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 25Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
getChrome Full support 38Edge Full support 12Firefox Full support 13IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support Yes
hasChrome Full support 38Edge Full support 12Firefox Full support 13IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support Yes
keysChrome Full support 38Edge Full support 12Firefox Full support 20IE No support NoOpera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 20Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
prototypeChrome Full support 38Edge Full support 12Firefox Full support 13IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support Yes
setChrome Full support 38Edge Full support 12Firefox Full support 13IE Partial support 11
Notes
Partial support 11
Notes
Notes Returns 'undefined' instead of the 'Map' object.
Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 14Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support Yes
sizeChrome Full support 38Edge Full support 12Firefox Full support 19
Notes
Full support 19
Notes
Notes From Firefox 13 to Firefox 18, the size property was implemented as a Map.prototype.size() method, this has been changed to a property in later versions conform to the ECMAScript 2015 specification.
IE Full support 11Opera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 19
Notes
Full support 19
Notes
Notes From Firefox 13 to Firefox 18, the size property was implemented as a Map.prototype.size() method, this has been changed to a property in later versions conform to the ECMAScript 2015 specification.
Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
valuesChrome Full support 38Edge Full support 12Firefox Full support 20IE No support NoOpera Full support 25Safari Full support 8WebView Android Full support 38Chrome Android Full support 38Edge Mobile Full support 12Firefox Android Full support 20Opera Android Full support 25Safari iOS Full support 8Samsung Internet Android Full support Yesnodejs Full support 0.12
@@iteratorChrome Full support YesEdge Full support YesFirefox Full support 36
Full support 36
No support 27 — 36
Notes Alternate Name
Notes A placeholder property named @@iterator is used.
Alternate Name Uses the non-standard name: @@iterator
No support 17 — 27
Notes Alternate Name
Notes A placeholder property named iterator is used.
Alternate Name Uses the non-standard name: iterator
IE No support NoOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 36
Full support 36
No support 27 — 36
Notes Alternate Name
Notes A placeholder property named @@iterator is used.
Alternate Name Uses the non-standard name: @@iterator
No support 17 — 27
Notes Alternate Name
Notes A placeholder property named iterator is used.
Alternate Name Uses the non-standard name: iterator
Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yesnodejs Full support 0.12
@@speciesChrome Full support 51Edge Full support 13Firefox Full support 41IE No support NoOpera Full support 38Safari Full support 10WebView Android Full support 51Chrome Android Full support 51Edge Mobile Full support 13Firefox Android Full support 41Opera Android Full support 41Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.5.0
Full support 6.5.0
Full support 6.0.0
Disabled
Disabled From version 6.0.0: this feature is behind the --harmony runtime flag.
@@toStringTagChrome Full support 44Edge No support NoFirefox Full support 51IE No support NoOpera No support NoSafari No support NoWebView Android Full support 44Chrome Android Full support 44Edge Mobile No support NoFirefox Android Full support 51Opera Android No support NoSafari iOS No support NoSamsung Internet Android Full support 4.0nodejs No support No

Legend

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Full support
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Partial support
No support  
No support
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.
Uses a non-standard name.
Uses a non-standard name.

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