map.js

Array.prototype.myMap = function(callback) {
  const arr = []

  for (let i = 0; i < this.length; i++) {
    arr.push(callback(this[i], i , this))   
  }

  return arr
}

const nums = [1, 6, 8]
console.log(nums.myMap(num => num * 2)) // [ 1, 12, 16 ]