JavaScript: How to Check if an Array has Duplicate Values

When dealing with arrays of values in JavaScript we sometimes want to determine if the array contains any duplicate values. Unfortunately, JavaScript arrays do not expose any built-in methods that can do this for us -- we have to write the implementation ourselves.

One approach to this problem might look like this:

function checkForDuplicates(array) {
  let valuesAlreadySeen = []

  for (let i = 0; i < array.length; i++) {
    let value = array[i]
    if (valuesAlreadySeen.indexOf(value) !== -1) {
      return true
  return false

This works, but in the worst case scenario where the only duplicate value occurs at the end of the array, this is not a very performant approach. We would have to iterate through the entire array (which could be huge!) only to realize at the last element that there is actually a non-unique value in the array.

Another approach that I recently learned harnesses the power of ES6 Sets.

In case you aren't familiar with Sets in JavaScript (I wasn't until recently!), here is the MDN definition:

Set objects are collections of values. You can iterate through the elements of a set in insertion order. A value in the Set may only occur once; it is unique in the Set's collection.

Read that last line one more time, as that is our secret sauce here. 'A value in the Set may only occur once; it is unique in the Set's collection.'

This fact means that we can convert our original array into a Set and then be confident that it only contains unique values. Once we've extracted all of the unique values from the array and stored them in our Set, we can compare the lengths of the array and the Set. If the lengths are not the same, it must follow that the array contained duplicate values!

Here's what that approach looks like:

function checkForDuplicates(array) {
  return new Set(array).size !== array.length

If the length of the Set and the array are not the same this function will return true, indicating that the array did contain duplicates. Otherwise, if the array and the Set are the same length the function will return false and we can be certain that the original array contained no duplicate values!

I really like this second approach for how concise and expressive it is, but you might run into browser support issues if you need to target older browsers so take that into consideration!