Sets, Maps, Dates & JSON

A Set stores unique values — duplicates are ignored:

const tags = new Set(['js', 'css', 'js']); tags.size → 2 tags.add('html'); tags.has('css') → true tags.delete('css');

The quickest way to remove duplicates from an array:

[...new Set([1, 2, 2, 3, 3])] → [1, 2, 3]

Loop with for...of. Sets also make membership checks fast on large collections.