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Sets in JavaScript

Atta on June 12, 2019

This post was originally published on attacomsian.com/blog. A Set is a special type of object in ES6 that lets you create a collection of uniqu...

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jtenner • Jun 12 '19

There is a lot of value when using a Set over an Array.

I learned yesterday that Set#has(item) performs an O(log n) algorithm to determine if the value exists in it. I was able to get a small speed increase in my javascript for it.

Good article!

Eugene Karataev • Jun 13 '19

AFAIK, Set.has implementation in V8 is really fast - it has O(1) time complexity.

jtenner • Jun 13 '19

It's entirely possible I learned wrong! I took the statement at face value, knowing I couldn't verify it myself. If that's the case, then I'm really happy I use Set all the time now.

Eugene Karataev • Jun 13 '19

Is it correct to say that Set is like array, but with unique items only and Map is like object where keys might be of any type (not limited to strings and symbols)?

Atta • Jun 13 '19

Well Set might look like an array but there are some notable differences. Apart from unique values, sets have different ways for initializing, accessing / adding / removing values.

Srđan Međo • Jul 20 '19

Sets are also unordered lists of elements, unlike arrays.

Eugene Karataev • Jul 20 '19

Why? We can use forEach method on Sets to iterate the collection by insertion order.

Srđan Međo • Jul 20 '19

Correct. Iteration is possible in the insertion order, but items are not indexed and they cannot be sorted unless converted to array beforehand. Maybe worth pointing out.

Eugene Karataev • Jul 20 '19

Agree. Sets also don't have array methods like map/filter/reduce and others. I just don't use Set/Map in my daily work and always forget what data structure is used for. By comparing Set to Array and Map to Object I want to create a mental connection in my head to remember the purpose of these data structures without looking in documentation.