Skip to content

JavaScript, how to find duplicates in an array

New Course Coming Soon:

Get Really Good at Git

How to find and remove duplicates in a JavaScript array

If you want to remove the duplicates, there is a very simple way, making use of the Set data structure provided by JavaScript. It’s a one-liner:

const yourArrayWithoutDuplicates = [ Set(yourArray)]

To find which elements are duplicates, you could use this “array without duplicates” we got, and and remove each item it contains from the original array content:

const yourArray = [1, 1, 2, 3, 4, 5, 5]

const yourArrayWithoutDuplicates = [ Set(yourArray)]

let duplicates = [...yourArray]
yourArrayWithoutDuplicates.forEach((item) => {
  const i = duplicates.indexOf(item)
  duplicates = duplicates
    .slice(0, i)
    .concat(duplicates.slice(i + 1, duplicates.length))

console.log(duplicates) //[ 1, 5 ]

Another solution is to sort the array, and then check if the “next item” is same to the current item, and put it into an array:

const yourArray = [1, 1, 2, 3, 4, 5, 5]

let duplicates = []

const tempArray = [...yourArray].sort()

for (let i = 0; i < tempArray.length; i++) {
  if (tempArray[i + 1] === tempArray[i]) {

console.log(duplicates) //[ 1, 5 ]

Note that this only works for primitive values, not objects. In the case of objects, you need a way to compare them.

Are you intimidated by Git? Can’t figure out merge vs rebase? Are you afraid of screwing up something any time you have to do something in Git? Do you rely on ChatGPT or random people’s answer on StackOverflow to fix your problems? Your coworkers are tired of explaining Git to you all the time? Git is something we all need to use, but few of us really master it. I created this course to improve your Git (and GitHub) knowledge at a radical level. A course that helps you feel less frustrated with Git. Launching May 21, 2024. Join the waiting list!
→ Get my JavaScript Beginner's Handbook
→ Read my JavaScript Tutorials on The Valley of Code
→ Read my TypeScript Tutorial on The Valley of Code

Here is how can I help you: