Array.prototype.map

Array.prototype.map returns a new array containing the results of calling a given function on every element of the calling array.

callbackFn: A function to execute for each element in the array. Its return value is added as a single element in the new array.

Pollyfill for map

/**
 * @callback callbackFn
 * @param {object} [thisArg]
 * @return {Array}
 */
Array.prototype.myMap = function (callbackFn, thisArg) {
  const len = this.length;
  const array = new Array(len);

  for (let k = 0; k < len; k++) {
    // Ignore index if value is not defined for index (e.g. in sparse arrays).
    if (Object.hasOwn(this, k)) {
      array[k] = callbackFn.call(thisArg, this[k], k, this);
    }
  }

  return array;
};

• Passing the index and array to the map callback.
• Calling the map callback with the correct this if thisArg is specified.
• Sparse arrays (e.g. [1, 2, , 4]). The empty values should be ignored while traversing the array.
• The thisArg doesn't do anything if the callback is defined as an arrow function as arrow functions don't have their own bindings to this.

Resources

• Array.prototype.map | MDN
• Array.prototype.map ECMAScript specification