emulating map method

myMap.js

// Array prototype method, however this is not safe, since it is added to the global scope
// definedProperty makes method non-enumerable as well as adding new property to in this case, to Array prototype
Object.defineProperty(Array.prototype, 'myMap', {
  // if callback is arrow function then context will be set to outer scope regardless of providing context argument
  value: function(callback, context) {
    'use strict';
    // will context to undefined if it is global object

    if (typeof callback !== 'function') {
      throw new TypeError(`${callback} is not a function`);
    }

    const length = this.length;
    let newArr = [];

    if (context !== undefined) {
      for (let i = 0; i < length; i++) {
        // check sparse array ex. [1, , ,2]
        if (i in this) {
          // call is faster than apply
          newArr[i] = callback.call(context, this[i], i, this);
          // not using push in case the array has empty slots
          // it's not possible to push empty slot https://stackoverflow.com/questions/56659890/possible-to-push-empty-slot-to-an-array

          // Instead I'm setting the index of the array to a value,
          // once a valid key/index is found after the empty slot,
          // the new result will be added at that specific index,
          // and previous uninitialized indices will be set to empty
        }
      }
    } else {
      // increase performance if there is no context or is window object
      for (let i = 0; i < length; i++) {
        if (i in this) {
          newArr[i] = callback(this[i], i, this);
        }
      }
    }

    // in case the whole array is sparse or last item is empty
    newArr.length = length;

    return newArr;
  },
});

// class alternative, safe since methods are locally scoped
class MyMethods {
  constructor() {}

  map(arr, callback, context) {
    if (typeof callback !== 'function') {
      throw new TypeError(`${callback} is not a function`);
    }

    const length = arr.length;
    const newArr = [];

    for (let i = 0; i < length; i++) {
      if (i in arr) {
        newArr[i] = callback.apply(context, [arr[i], i, arr]);
      }
    }

    if (newArr.length < length) {
      newArr.length = length;
    }

    return newArr;
  }
}