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How to create an array of unique values in JavaScript using Sets

Claire Parker-Jones on May 08, 2018

TL;DR let uniqueArray = [...new Set([5,5,2,2,2,4,2])]; // [5,2,4] Enter fullscreen mode Exit fullscreen mode ...

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Xing Wang • May 9 '18

Good highlight this feature.

However, it maybe worth pointing out that with Set, you can't control the equality operator.

It uses '===', which only work only off same actual object or same value for a primitive.

Which limits the usage somewhat compares to other methods such as map or filter.

Tommy • May 16 '23

If you find yourself needing to push unique items to an existing array that was declared with const (i.e., you cannot use new Set()), then here are two patterns you can use:

// Given this...
const arr = ['A', 'B', 'C'];
const itemsToAdd = ['C', 'D', 'E'];

// Both methods will produce arr = ['A', 'B', 'C', 'D', 'E']

// Method 1:
itemsToAdd.forEach((i) => {
  if (!arr.includes(i)) {
    arr.push(i);
  }
});

// Method 2:
arr.push(...itemsToAdd.filter((i) => !arr.includes(i)));

Method 1 is about 30% faster in my Chrome:
jsbench.me/jzlhpp4thr/1

sebkolind | ⛺ Tent.js • May 9 '18

I will implement this right away in a project I am working on. Although I like the Array.from() better, since it's easier to read. I like easy-to-read code :) Thank you for sharing!

Ashish Mehra • May 9 '18 • Edited

I used to filter and map over an array to remove duplicates. Thanks for sharing this method, Mam.

Jochem Stoel • Oct 19 '18

@claireparker I have been hanging around in this neighborhood for little over a year now and appreciated a lot of quality content from many different sources but this is the first time I am Googling something and it leads to finding the answer here on this website.

You have a new follower.

Seth T • May 13 '18

Is there any difference in performance between Array.from and using the spread operator?

KimLongUn • Mar 27 '19

"They both accomplish the same thing here so the choice is yours!"

So i dont know if there is some missunderstanding here but as far as i see it they dont work the same as in my case using the spread operator returns

[Set(7)]
0: Set(7) {"anthropological", "security", "surveillance", "technology", "biology", …}

while using Array.from returns

(7) ["anthropological", "security", "surveillance", "technology", "biology", "extra-algorithmic experience", "randomness"]

Neocortexxx • Oct 30 '19 • Edited

"They both accomplish the same thing here so the choice is yours!"
Well, actually she refers to the spread operator ('...') and Array.from.
In both cases you still have to use new Set.