Simple implementation of array.prototype.map

build_your_own_map.js

// overriding properties on Array.prototype is a bad idea. This is given for educational purposes only :D
Array.prototype.map = function(projectionFn) {
 let retVal = [];
  
 for (let i = 0; i < this.length; i++) {
   retVal.push(projectionFn(this[i])); 
 }
  
  return retVal;
}