How to check if an array contains duplicate values in JavaScript?

Checking if a JavaScript Array contains duplicate values is something that might come in handy. For instance, I frequently use this in tests to perform verifications.

Let's now see how to do it. Considering the following 2 arrays:

const array1 = ['one', 'two', 'three'];
const array2 = ['one', 'two', 'two', 'three'];

The first one, array1, contains no duplicates. However, the second, array2, contains the item two twice.

In an ES2015/ES6 compatible environment, which is very likely if you are using any modern browser or supported Node.js version, checking for duplicates is as easy as running the following snippet:

function containsDuplicates(array) {
  return array.length !== new Set(array).size;
}

References

• JavaScript | MDN: Set - Remove duplicate elements from an array
• JavaScript | MDN: Array