Handmade Array.prototype.map()

custom_map.js

function customMap(arr, callback, thisArgs) {
  let newArr = [];
  
  for (let i = 0; i < arr.length; i++) {
    newArr.push(callback.call(thisArgs, arr[i]));
  }
  
  return newArr;
}

// Или так 
Array.prototype._map = function(fn) {
  let res = [];

  for (let i = 0; i < this.length; i++) {
    res.push(fn(this[i]));
  }

  return res;
}

// Filter
Array.prototype._filter = function(fn, args) {
  let res = [];

  for (let i = 0; i < this.length; i++) {
    if (fn.call(null, this[i])) {
      res.push(this[i]);
    }
  }

  return res;
}