[ES6] Set && WeakSet

 Limitations With Array

Arrays don't enforce uniqueness of items. Diplicate entries are allowed.

Using Sets 

let tags = new Set() ;
tags.add('Javascript');
tags.add('Programming');
tags.add('Web');

console.log(`Total items ${tags.size}`);

Sets and for...of

let tags = new Set();

tags.add("JavaScript");
tags.add("Programming");
tags.add("Web");

for( let tag of tags ){
  console.log(`Tag: ${tag}`);
}

Sets and Destructuring 

let tags = new Set();

tags.add("JavaScript");
tags.add("Programming");
tags.add("Web");

let [first] = tags;

console.log( `First tag: ${first}` );

WeakSets

The WeakSet is a more memory efficient type of Set where only objets are allowed to be stored.

WeakSets in Action 

let allPosts = new WeakSet();

let post1 = { title: "ES2015" };
let post2 = { title: "CoffeeScript" };
let post3 = { title: "TypeScript" };

allPosts.add( post1 );
allPosts.add( post2 );
allPosts.add( post3 );

How can we query the allPosts WeakSet to determine whether it has the post2 object?

allPosts.has(post2);