How do higher-order functions like `map()` and `reduce()` receive their data?

Question

I'm trying to write my own higher order function right now and I want to know how functions like map() and reduce() access the array they are being applied to. And not just for arrays either, but with any higher order function like toString() or toLowerCase().

array.map()
^^^ // How do I get this data when I am writing my own higher order function?

array.myOwnFunction(/* data??? */)

I hope this makes sense. I'm sure the answer is out there already, but I'm struggling to know what to search for to find the information.

Answer 1

You can add it to the Array prototype like :

Array.prototype.myOwnFunction = function() {
  for (var i = 0; i < this.length; i++) {
    this[i] += 1;
  }

  return this;
};

const array = [1, 2, 3];

const result = array.myOwnFunction();

console.log(result);

Answer 2

Check the polyfill for Array.prototype.map(), this line in particular:

//  1. Let O be the result of calling ToObject passing the |this| 
//    value as the argument.
var O = Object(this);

Simplifying, this is where the values are received.