How to Initialize a Set with Values in JavaScript

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Borislav Hadzhiev

Last updated: Oct 27, 2021

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Initialize a Set with Values in JavaScript #

To initialize a Set with values, pass an iterable to the Set constructor. When an iterable is passed to the Set constructor, all elements get added to the new Set. The most common iterables to initialize a Set with are - array, string and another Set.

index.js
const set1 = new Set(['one', 'one', 'two', 'three']); console.log(set1); // 👉️ {'one', 'two', 'three'} const set2 = new Set('hello'); console.log(set2); // 👉️ {'h', 'e', 'l', 'o'} const set3 = new Set([...set1, 'four']); console.log(set3); // 👉️ {'one', 'two', 'three', 'four'} set3.add('five'); set3.add('six'); // 👇️ {'one', 'two', 'three', 'four', 'five', 'six'} console.log(set3);

In the first example, we passed an array to the Set() constructor.

All of the elements in the array got added to the new Set, except for the duplicates.

Set objects only store unique values.

In our second example, we initialized the Set with a string. The string got split on every character and added to the new Set.

The l character is contained twice in the string hello, however it is only contained a single time in the Set.

Note that the character has to be the same case to be considered duplicate. For example, both ts from the string Test will be added to the Set.

index.js
const set4 = new Set('Test'); console.log(set4); // 👉️ {'T', 'e', 's', 't'}

In our third Set, we used the spread operator (...) to unpack the values from the first set and add an additional value.

Sets are also iterable, just like arrays and strings.

An alternative approach is to create an empty set and manually add values to it.

index.js
const set5 = new Set(); set5.add('one'); set5.add('two'); set5.add('three'); console.log(set5); // 👉️ {'one', 'two', 'three'}

Further Reading #

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