myMap

Array.prototype.mySum = function(callback) {
  const vals = this;

  const res = [];

  for (var i = 0; i < vals.length; i++) {
    res.push(callback(this[i]));
  }

  return res;
}

function cb(input) {
  return input + 1;
}

const a = [1,2,3].mySum(x => cb(x));