How to Check if a JavaScript Array Contains Duplicate Values?

Sometimes, we want to check if a JavaScript array contains duplicate values.

In this article, we’ll look at how to check if a JavaScript array contains duplicate values.

To check if a JavaScript array contains duplicate values, we can use the JavaScript array some and indexOf methods.

For instance, we write:

const arr = [11, 22, 11, 22];
const hasDuplicate = arr.some((val, i) => arr.indexOf(val) !== i);
console.log(hasDuplicate)

to check if arr has duplicate values.

We call some with a callback to check if some array entries matches the condition returned by the callback.

indexOf returns the index of the first element that matches a given value.

So if indexOf returns a value that isn’t equal to the index i, we know that the value is a duplicate.

Therefore, the console log should log true.

To check if a JavaScript array contains duplicate values, we can use the JavaScript array some and indexOf methods.