JavaScript, how to find duplicates in an array

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 = [...new 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 = [...new 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]) {  duplicates.push(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.