Create Map and Reduce from scratch in JS.

js_map_and_reduce_from_scratch.js

Array.prototype.reduce = function(fn, acc) {
  if (acc === undefined && this.length !== 0)
    return this.slice(1).reduce(fn, this[0]);

  if (this.length === 0) return acc;

  return this.slice(1).reduce(fn, fn(acc, this[0]));
};


Array.prototype.map = function(fn) {
  return this.reduce(function(acc, el) {
    return acc.concat(fn(el));
  }, []);
};