To get all non-unique values in an array with JavaScript, we call the filter and indexOf methods.
For instance, we write
const noDups = [1, 2, 2, 4, 3, 4].filter((e, i, a) => a.indexOf(e) !== i);
to call filter with a callback that checks if a.indexOf returns an index that’s different from i.
If this is true, that means there’s a duplicate of e since indexOf returns the first index of e.
