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151e92adb50-113e046b.md
{{JSRef}} {{harmony}}

The Map object is a simple key/value map. Any value (both objects and {{Glossary("Primitive", "primitive values")}}) may be used as either a key or a value.

Syntax

new Map([iterable])

Parameters

iterable
Iterable is an Array or other iterable object whose elements are key-value pairs (2-element Arrays). Each key-value pair is added to the new Map. null is treated as undefined.

Description

A Map object iterates its elements in insertion order — a {{jsxref("Statements/for...of", "for...of")}} loop returns an array of [key, value] for each iteration.

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 6 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 {{geckoRelease("29")}} ({{bug("952870")}}) and a recent nightly Chrome.

Objects and maps compared

{{jsxref("Object", "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. This could be bypassed by using map = Object.create(null) since ES5, but was seldomly done.
  • The keys of an Object are {{jsxref("String", "Strings")}} and {{jsxref("Symbol", "Symbols")}}, where they can be any value for a Map.
  • You can get the size of a Map easily while you have to manually keep track of size for an Object.

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.

Properties

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

Map instances

All Map instances inherit from {{jsxref("Map.prototype")}}.

Properties

{{page('en-US/Web/JavaScript/Reference/Global_Objects/Map/prototype','Properties')}}

Methods

{{page('en-US/Web/JavaScript/Reference/Global_Objects/Map/prototype','Methods')}}

Examples

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);
}
// Will show 2 logs; first with "0 = zero" and second with "1 = one"

for (var key of myMap.keys()) {
  console.log(key);
}
// Will show 2 logs; first with "0" and second with "1"

for (var value of myMap.values()) {
  console.log(value);
}
// Will show 2 logs; first with "zero" and second with "one"

for (var [key, value] of myMap.entries()) {
  console.log(key + " = " + value);
}
// Will show 2 logs; first with "0 = zero" and second with "1 = one"

Iterating Maps with forEach()

Maps can be iterated using the forEach() method:

myMap.forEach(function(value, key) {
  console.log(key + " = " + value);
}, myMap)
// 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"]

Specifications

Specification Status Comment
{{SpecName('ES6', '#sec-map-objects', 'Map')}} {{Spec2('ES6')}} Initial definition.

Browser compatibility

{{CompatibilityTable}}

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

{{ CompatChrome(38) }} [1]

 {{ CompatGeckoDesktop("13") }} 11 25 7.1
Constructor argument: new Map(iterable) {{ CompatChrome(38) }} {{ CompatGeckoDesktop("13") }} {{CompatNo}} 25 {{CompatNo}}
iterable {{ CompatChrome(38) }} {{ CompatGeckoDesktop("17") }} {{CompatNo}} 25 7.1
Map.clear() {{ CompatChrome(31) }}
{{ CompatChrome(38) }}		 {{CompatGeckoDesktop("19")}} 11 25 7.1
Map.keys(), Map.values(), Map.entries() {{ CompatChrome(37) }}
{{ CompatChrome(38) }}		 {{CompatGeckoDesktop("20")}} {{CompatNo}} 25 7.1
Map.forEach() {{ CompatChrome(36) }}
{{ CompatChrome(38) }}		 {{CompatGeckoDesktop("25")}} 11 25 7.1
Key equality for -0 and 0 {{ CompatChrome(34) }}
{{ CompatChrome(38) }}		 {{CompatGeckoDesktop("29")}} {{CompatNo}} 25 {{CompatNo}}
Constructor argument: new Map(null) {{CompatVersionUnknown}} {{CompatGeckoDesktop("37")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Monkey-patched set() in Constructor {{CompatVersionUnknown}} {{CompatGeckoDesktop("37")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Map[@@species] {{CompatUnknown}} {{CompatGeckoDesktop("41")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Map() without new throws {{CompatUnknown}} {{ CompatGeckoDesktop("42") }} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support {{CompatNo}} {{ CompatChrome(38) }} [1] {{ CompatGeckoMobile("13") }} {{CompatNo}} {{CompatNo}} 8
Constructor argument: new Map(iterable) {{CompatNo}} {{ CompatChrome(38) }} {{ CompatGeckoMobile("13") }} {{CompatNo}} {{CompatNo}} {{CompatNo}}
iterable {{CompatNo}} {{CompatNo}} {{ CompatGeckoMobile("17") }} {{CompatNo}} {{CompatNo}} 8
Map.clear() {{CompatNo}} {{ CompatChrome(31) }}
{{ CompatChrome(38) }}		 {{CompatGeckoMobile("19")}} {{CompatNo}} {{CompatNo}} 8
Map.keys(), Map.values(), Map.entries() {{CompatNo}} {{ CompatChrome(37) }}
{{ CompatChrome(38) }}		 {{CompatGeckoMobile("20")}} {{CompatNo}} {{CompatNo}} 8
Map.forEach() {{CompatNo}} {{ CompatChrome(36) }}
{{ CompatChrome(38) }}		 {{CompatGeckoMobile("25")}} {{CompatNo}} {{CompatNo}} 8
Key equality for -0 and 0 {{CompatNo}} {{ CompatChrome(34) }}
{{ CompatChrome(38) }}		 {{CompatGeckoMobile("29")}} {{CompatNo}} {{CompatNo}} {{CompatNo}}
Constructor argument: new Map(null) {{CompatUnknown}} {{CompatVersionUnknown}} {{CompatGeckoMobile("37")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Monkey-patched set() in Constructor {{CompatUnknown}} {{CompatVersionUnknown}} {{CompatGeckoMobile("37")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Map[@@species] {{CompatUnknown}} {{CompatUnknown}} {{CompatGeckoMobile("41")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}
Map() without new throws {{CompatUnknown}} {{CompatUnknown}} {{CompatGeckoMobile("42")}} {{CompatUnknown}} {{CompatUnknown}} {{CompatUnknown}}

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

See also

Raw
note-info.json
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