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Implement Array.map

Array.map

The map() method creates a new array with the results of calling a provided function on every element in the calling array.

Question

Implement the Array.map method

Thought process Pseudocode

  1. A function which takes a function as an argument
  2. The function argument must be passed the array index, the item and the context

Solution

/**
 * Map function is attached to the array prototype
 * It takes in a callback and passes every item in the array
 * through the callback, the result is pushed into a new array
 * which is returned
 */
/**
 * The callback gets 3 arguments
 * the item, the index and the  execution context
 */
Array.prototype.newMap = function (callback) {
  const result = [];
  for (let index = 0; index < this.length; index++) {
    // This is primarily to check if the item
    // exists in the array, 
    if (this.indexOf(this[index]) > -1) {
      result[index] = callback(this[index], index, this)
    }
  }
  return result
}
// example
const numbers = [1, 2, 3, 4]
numbers[10] = 34;
const double = numbers.newMap((item, index) => {
  return item * 2
})
console.log(double)

Caveat

The reason for the if condition is basically, to ensure the item is present in the array, Array.map does not alter the index of the items in the resultant array, hence in the case above there will be 6 entries which will be emtpty