To check for duplicate strings in JavaScript array, we call the filter method.

For instance, we write

const strArray = ["q", "w", "w", "w", "e", "i", "u", "r"];
const findDuplicates = (arr) =>
  arr.filter((item, index) => arr.indexOf(item) !== index);

to define the findDuplicates method.

In it, we call arr.filter with a callback that checks if the first index of item isn’t index.

If they’re different, then the item is a duplicate.