Understanding .filter(); Seek and Destroy

Can anyone explain the == -1 at the end of this callback function?

function destroyer(arr) {
  var args = Array.from(arguments);
  return arr.filter(function(element) {
    return args.indexOf(element) == -1;

destroyer([1, 2, 3, 1, 2, 3], 2, 3);

args.indexOf(element) returns -1 if element is not found anywhere in the args array.

args.indexOf(element) == -1 will evaluate to true if element is not in the args array. This expression is equivalent to !args.includes(element).

Read more about indexOf() and includes().

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indexOf returns -1 if element (from arr) is not present in args. So the whole equality check is true for elements that are not in args and false otherwise. Filter only retain elements for which the callback returns true.

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I assumed it was related to .filter(); It all makes sense now. Thank you.