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Map 物件是簡單的鍵/值映射。 鍵和值可以是任意的值(物件或原始型別)。


new Map([iterable])


Iterable 是一個陣列或其他iterable物件,該物件的元素皆是鍵/值成對的(如兩個元素的陣列)。成對的鍵/值會被新增到新的Map。null會被作為undefined


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 迴圈會在每次回傳一個[key, value]陣列。

It should be noted that a Map that is a map of an object, especially a dictionary of dictionaries, will only map to the object's insertion order -- which is random and not ordered.  

Key equality

Key equality is based on the "same-value" algorithm: NaN is considered the same as NaN (even though NaN !== NaN) and all other values are considered equal according to the semantics of the === operator. In earlier versions of the ECMAScript 2015 draft -0 and +0 were considered distinct (even though -0 === +0), this has been changed in later versions and has been adapted in Gecko 29 (Firefox 29 / Thunderbird 29 / SeaMonkey 2.26) (bug 952870) and a recent nightly Chrome.

Objects and maps compared

Objects are similar to Maps in that both let you set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Because of this (and because there were no built-in alternatives), Objects have been used as Maps historically; however, there are important differences between Objects and Maps that make using a Map better:

  • An Object has a prototype, so there are default keys in the map that could collide with your keys if you're not careful. This could be bypassed by using map = Object.create(null) since ES5, but was seldom done.
  • The keys of an Object are Strings and Symbols, whereas they can be any value for a Map, including functions, objects, and any primitive.
  • You can get the size of a Map easily with the size property, while the size of an Object must be determined manually. 

This does not mean you should use Maps everywhere, objects still are used in most cases. Map instances are only useful for collections, and you should consider adapting your code where you have previously used objects for such. Objects shall be used as records, with fields and methods.
If you're still not sure which one to use, ask yourself the following questions:

  • Are keys usually unknown until run time, do you need to look them up dynamically?
  • Do all values have the same type, and can be used interchangeably?
  • Do you need keys that aren't strings?
  • Are key-value pairs often added or removed?
  • Do you have an arbitrary (easily changing) amount of key-value pairs?
  • Is the collection iterated?

Those all are signs that you want a Map for a collection. If in contrast you have a fixed amount of keys, operate on them individually, and distinguish between their usage, then you want an object.


The value of the length property is 0.
get Map[@@species]
The constructor function that is used to create derived objects.
Represents the prototype for the Map constructor. Allows the addition of properties to all Map objects.

Map instances

All Map instances inherit from Map.prototype.


Returns the function that created an instance's prototype. This is the Map function by default.
Returns the number of key/value pairs in the Map object.


Removes all key/value pairs from the Map object.
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.
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.
Returns the value associated to the key, or undefined if there is none.
Returns a boolean asserting whether a value has been associated to the key in the Map object or not.
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.
Returns a new Iterator object that contains the values for each element in the Map object in insertion order.
Returns a new Iterator object that contains an array of [key, value] for each element in the Map object in insertion order.


Using the Map object

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 () {}

Using NaN as Map keys

NaN can also be used as a key. Even though every NaN is not equal to itself (NaN !== NaN is true), the following example works, because NaNs are indistinguishable from each other:

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"

Iterating Maps with for..of

Maps can be iterated using a for..of loop:

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()) {
// 0
// 1

for (var value of myMap.values()) {
// zero
// one

for (var [key, value] of myMap.entries()) {
  console.log(key + " = " + value);
// 0 = zero
// 1 = one

Iterating Maps with forEach()

Maps can be iterated using the forEach() method:

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

Relation with Array objects

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 spread operator to transform a map into a 2D key-value Array.
console.log(uneval([...myMap])); // Will show you exactly the same Array as kvArray

// Or use the spread operator on the keys or values iterator to get 
// an array of only the keys or values
console.log(uneval([...myMap.keys()])); // Will show ["key1", "key2"]


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.

Browser compatibility

We're converting our compatibility data into a machine-readable JSON format. This compatibility table still uses the old format, because we haven't yet converted the data it contains. Find out how you can help!

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari
Basic support

38 [1]

12 13 (13) 11 25 7.1
Constructor argument: new Map(iterable) 38 12 13 (13) No support 25 9
iterable 38 12 17 (17) No support 25 7.1
Map.clear() 31
12 19 (19) 11 25 7.1
Map.keys(), Map.values(), Map.entries() 37
12 20 (20) No support 25 7.1
Map.forEach() 36
12 25 (25) 11 25 7.1
Key equality for -0 and 0 34
12 29 (29) No support 25 9
Constructor argument: new Map(null) (Yes) 12 37 (37) 11 (Yes) 9
Monkey-patched set() in Constructor (Yes) 12 37 (37) No support (Yes) 9
Map[@@species] 51 13 41 (41) No support 38 10
Map() without new throws (Yes) 12 42 (42) 11 (Yes) 9
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support No support 38 [1] 13.0 (13) No support No support 8
Constructor argument: new Map(iterable) No support 38 13.0 (13) No support No support 9
iterable No support No support 17.0 (17) No support No support 8
Map.clear() No support 31
19.0 (19) No support No support 8
Map.keys(), Map.values(), Map.entries() No support 37
20.0 (20) No support No support 8
Map.forEach() No support 36
25.0 (25) No support No support 8
Key equality for -0 and 0 No support 34
29.0 (29) No support No support No support
Constructor argument: new Map(null) ? (Yes) 37.0 (37) ? ? 9
Monkey-patched set() in Constructor ? (Yes) 37.0 (37) ? ? 9
Map[@@species] ? ? 41.0 (41) ? ? 10
Map() without new throws 5.1 ? 42.0 (42) ? ? 9

[1] Starting with Chrome 31, the feature was available behind a preference. In chrome://flags, activate the entry “Enable Experimental JavaScript”.

See also


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