Javascript / How does the function know about nature of value, index and self?

Question

I have tried to understand this function :

function onlyUnique(value, index, self) {
    return self.indexOf(value) === index;
  }
  
  var a = ['a', 1, 'a', 2, '1'];
  var unique = a.filter(onlyUnique); // returns ['a',1,2,'1']

I wonder how onlyUnique knows about value, index and self being indeed the value, the index, and the array ? Is this related to the filter ?

Probably you should check the polyfill for `filter` method to get better understanding. — Abhay Srivastav, Aug 02 '22 at 14:05
@AbhaySrivastav Why not just see the [documentation for filter](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter) — phuzi, Aug 02 '22 at 14:06
When a function is called, the arguments are passed values in order. The names the function uses for them internally can be anything you like. — Quentin, Aug 02 '22 at 14:07

score 4 · Accepted Answer · answered Aug 02 '22 at 14:05

As you can see in the MDN docs filter function can take multiple arguments. The rest is purely how javascript functions work.

function onlyUnique(value, index, self) {
  return self.indexOf(value) === index;
}
  
var a = ['a', 1, 'a', 2, '1'];
var unique = a.filter(onlyUnique); // returns ['a',1,2,'1']

is simply equivalent to:

function onlyUnique(value, index, self) {
  return self.indexOf(value) === index;
}
  
var a = ['a', 1, 'a', 2, '1'];
var unique = a.filter((value, index, self) => onlyUnique(value, index, self)); // returns ['a',1,2,'1']

Javascript / How does the function know about nature of value, index and self?

1 Answers1