ES6 cheatsheet  —  Set & WeakSet

Headshot Mihai Serban

Serban Mihai / 20 October 2018

~2 min read

ES6 Set

A Set is a collection for unique values. The values can be primitives or object references.

let set = new Set();  
set.add({ key: 'value' });  
console.log(set); // Set {1, '1', Object {key: 'value'}}

Most importantly is that it does not allow duplicate values, one good use if to remove duplicate values from an array:

[ Set([1, 2, 3, 1, 2, 3]) ] //[1, 2, 3]

Iteration using built-in method forEach and for..of:

// forEach  
let set = new Set([1, '1', { key: 'value' }]);  
set.forEach(function (value) {  
  // 1  
  // '1'  
  // Object {key: 'value'}  

// for..of  
let set = new Set([1, '1', { key: 'value' }]);  
for (let value of set) {  
  // 1  
  // '1'  
  // Object {key: 'value'}  

Similar to Map, Set provides us with methods such as has(), delete(), clear().

Find more details about Set here


Like a WeakMap, WeakSet is a Set that doesn’t prevent its values from being garbage-collected. It has simpler API than WeakMap, because has only three methods:

new WeakSet([iterable])  
WeakSet.prototype.add(value)    : any  
WeakSet.prototype.has(value)    : boolean  
WeakSet.prototype.delete(value) : boolean

Important thing to note WeakSet is a collection that can‘t be iterated and whose size cannot be determined.

Find more details about WeakSet here

You can find a more complete ES6 cheetsheet on my Github page.

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