3 Ways to Remove Duplicates in JavaScript Arrays

This blog will run you through 3 ways to filter unique values from an array in JavaScript, removing duplicate values.

This blog will run you through 3 ways to filter unique values from an array in JavaScript, removing duplicate values.

1. Use filter and indexOf to get only the first instance of each value

This approach involves running a filter** over the array** — for each value, we only add it to the unique array if it is the first item with that value in the array, i.e. if arr.indexOf(value) === index:

const arr = ['a', 'b', 'a', 'b', 'c'];

const uniqueArr = arr.filter((value, index) => {
  return arr.indexOf(value) === index;
});
console.log(uniqueArr); // ['a', 'b', 'c']

2. Use a map object to store unique values

This second approach relies on JavaScript map objects, which cannot have duplicate keys. It involves creating an empty mapObj, and adding each value of the array to mapObj as a key. The final unique array can then be retrieved using Object.keys(mapObj):

const arr = ['a', 'b', 'a', 'b', 'c'];

const mapObj = {};
arr.forEach(a => { 
  mapObj[a] = true;
});
const uniqueArr = Object.keys(mapObj);
console.log(uniqueArr); // ['a', 'b', 'c']

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