Collections (JavaScript)

You can use the collection objects Map, Set, and WeakMap to store values and objects. These objects provide convenient methods for adding and retrieving members by using either a key or a value instead of an index. To access members of a collection by using an index, use an Array object. For more information, see Using Arrays.


Map, Set, and WeakMap are not supported in browser versions before Internet Explorer 11. For more information about version support, see Version Information.

Using collections

The Map and WeakMap objects store key/value pairs and enable you to add, retrieve, and remove members by using the key. The key and the value may be of any type. The Set object stores values of any type.

The Map and Set objects enable you to enumerate collection members by using the forEach method and to check the size of the collection by using the size method. The WeakMap object, in contrast, is not enumerable. For this collection, the key references are held weakly. Use WeakMap if you want the garbage collector to determine whether the app has to retain each member of the collection in memory. For example, this may be useful in caching scenarios where cached objects are very large and you don't want to hold objects in memory unnecessarily. In some scenarios, you can use this object to prevent memory leaks.

The following example shows how to use the Map object. In this example, you access members by using both get and forEach. The callback function in forEach can take up to three parameters, which provide the value of the current collection element, the key of the current element, and the collection object itself.

var m = new Map();  
m.set(1, "black");  
m.set(2, "red");  
m.set("colors", 2);  
m.set({x:1}, 3);  

document.write("<br />");  

m.forEach(function (value, key, mapObj) {  
    document.write(item.toString() + "<br />");  

// Output:  
// red  

// black  
// red  
// 2  
// 3  

The use of WeakMap is similar to Map, except that you can retrieve members only by using get. For an example, see the WeakMap object.

The following example shows how to use the Set object. In this example, the callback function takes one parameter, which is the value of the current collection element.

var s = new Set();  
s.add("Thomas Jefferson");  
s.add("founding father");  

s.forEach(function (value) {  
    document.write(item.toString() + ", ");  

// Output:  
// Thomas Jefferson, 1776, founding father,  

See Also

Advanced JavaScript