Keyed collections

This chapter introduces collections of data which are ordered by a key; Map and Set objects contain elements which are iterable in the order of insertion.


Map object

ECMAScript 6 introduces a new data structure to map values to values. A Map object is a simple key/value map and can iterate its elements in insertion order

The following code shows some basic operations with a Map. See also the Map reference page for more examples and the complete API. You can use a for...of loop to return an array of [key, value] for each iteration.

var sayings = new Map();
sayings.set("dog", "woof");
sayings.set("cat", "meow");
sayings.set("elephant", "toot");
sayings.size; // 3
sayings.get("fox"); // undefined
sayings.has("bird"); // false
sayings.has("dog"); // false

for (var [key, value] of sayings) {
  console.log(key + " goes " + value);
// "cat goes meow"
// "elephant goes toot"

sayings.size; // 0 

Object and Map compared

Traditionally, objects have been used to map strings to values. Objects allow you to set keys to values, retrieve those values, delete keys, and detect whether something is stored at a key. Map objects, however, have a few more advantages that make them better maps.

  • The keys of an Object are Strings, where they can be of 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.
  • The iteration of maps is in insertion order of the elements.
  • An Object has a prototype, so there are default keys in the map. (this can be bypassed using map = Object.create(null)).

These three tips can help you to decide whether to use a Map or 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 maps in case if there is a need to store primitive values as keys because object treats each key as a string whether it's a number value, boolean value or any other primitive value.
  • Use objects when there is logic that operates on individual elements.

WeakMap object

The WeakMap object is a collection of key/value pairs in which the keys are objects only and the values can be arbitrary values. The object references in the keys are held weakly meaning that they are target of garbage collection (GC) if there is no other reference to the object anymore. The WeakMap API is the same as the Map API.

One difference to Map objects is that WeakMap keys are not enumerable (i.e. there is no method giving you a list of the keys). If they were, the list would depend on the state of garbage collection, introducing non-determinism.

For more information and example code, see also "Why WeakMap?" on the WeakMap reference page.

One use case of WeakMap objects is to store private data for an object or to hide implementation details. The following example is from Nick Fitzgerald blog post "Hiding Implementation Details with ECMAScript 6 WeakMaps". The private data and methods belong inside the object and are stored in the privates WeakMap object. Everything exposed on the instance and prototype is public; everything else is inaccessible from the outside world because privates is not exported from the module

const privates = new WeakMap();

function Public() {
  const me = {
    // Private data goes here
  privates.set(this, me);

Public.prototype.method = function () {
  const me = privates.get(this);
  // Do stuff with private data in `me`...

module.exports = Public;


Set object

Set objects are collections of values. You can iterate its elements in insertion order. A value in a Set may only occur once; it is unique in the Set's collection.

The following code shows some basic operations with a Set. See also the Set reference page for more examples and the complete API.

var mySet = new Set();
mySet.add("some text");

mySet.has(1); // true
mySet.size; // 2

for (let item of mySet) console.log(item);
// 1
// "some text"

Converting between Array and Set

You can create an Array from a Set using Array.from or the spread operator. Also, the Set constructor accepts an Array to convert in the other direction. Note again that Set objects store unique values, so any duplicate elements from an Array are deleted when converting.


mySet2 = new Set([1,2,3,4]);

Array and Set compared

Traditionally, a set of elements has been stored in arrays in JavaScript in a lot of situations. The new Set object, however, has some advantages:

  • Checking whether an element exists in an collection using indexOf for arrays is slow.
  • Set objects let you delete elements by their value. With an array you would have to splice based on a element's index.
  • The value NaN cannot be found with indexOf in array.
  • Set objects store unique values, you don't have to keep track of duplicates by yourself.

WeakSet object

WeakSet objects are collections of objects. An object in the WeakSet may only occur once; it is unique in the WeakSet's collection and objects are not enumerable.

The main differences to the Set object are:

  • In contrast to Sets, WeakSets are collections of objects only and not of arbitrary values of any type.
  • The WeakSet is weak: References to objects in the collection are held weakly. If there is no other reference to an object stored in the WeakSet, they can be garbage collected. That also means that there is no list of current objects stored in the collection. WeakSets are not enumerable.

The use cases of WeakSet objects are limited. They will not leak memory so it can be safe to use DOM elements as a key and mark them for tracking purposes, for example.

Key and value equality of Map and Set

Both, the key equality of Map objects and the value equality of Set objects, are based on the "same-value-zero algorithm":

  • Equality works like the identity comparison operator ===.
  • -0 and +0 are considered equal.
  • NaN is considered equal to itself (contrary to ===).


© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.

Collections Guide JavaScript Map set