Sets in JavaScript

Atta on June 12, 2019

A Set is a special type of object in ES6 that lets you create a collection of unique values. Each value appears only once in the set. The values st... [Read Full]
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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!


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


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.


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)?


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.


Sets are also unordered lists of elements, unlike arrays.

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

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.

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.


Are set values unique, I can't remember, but if they are passing a string would be destructive, hello world would be helo wrd


Yes, Set values are unique. It will filter-out the duplicate characters if you pass a string to create a set.

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