Map

Introduction

Map オブジェクトはシンプルなキーバリューマップです。 キーとバリューにあらゆる値（オブジェクトとプリミティブ値）が使用できます。

Key equality is based on the "same-value" algorithm: NaN is considered the same as NaN (even though NaN !== NaN), -0 and +0 are considered distinct (even though -0 === +0), and all other values are considered equal according to the semantics of the === operator.

API

A Map object can iterate its elements in insertion order - a for..of loop will return an array of [key, value] for each iteration.

Examples

var myMap = new Map();

var keyObj = {},
    keyFunc = function () {},
    keyString = "a string";

// 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 () {}

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"

Also note that JavaScript has two zero values, +0 and -0. These two zero values are treated as different keys in Maps:

var myMap = new Map();
myMap.set(0, "positive zero");
myMap.set(-0, "negative zero");

0 === -0; // true

myMap.get(-0); // "negative zero"
myMap.get(0);  // "positive zero"

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) {
  alert(key + " = " + value);
}
// Will show 2 alerts; first with "0 = zero" and second with "1 = one"

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, 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. However, this can be bypassed using map = Object.create(null).
• The keys of an Object are Strings, 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.

Use maps over objects when keys are unknown until run time, and when all keys are the same type and all values are the same type.

Use objects when there is logic that operates on individual elements.

Browser compatibility

[1] The feature is available behind a preference. In chrome://flags, activate the entry “Enable Experimental JavaScript”.

See also

• Map and Set bug at Mozilla
• ECMAScript Harmony proposal