Discussion on: Let's create our map method

Very nice as an example, just remember is an example that doesn't fully replace the builtin function, so don't use it in production.

As a completion, here is the lodash map source code

function map(array, iteratee) {
  let index = -1
  const length = array == null ? 0 : array.length
  const result = new Array(length)

  while (++index < length) {
    result[index] = iteratee(array[index], index, array)
  }
  return result
}

Some side notes about the differences between the custom maps:

• it pre-allocates the result array (we already know its length) const result = new Array(length)
• it uses const where the values don't change (like the result and length)
• both implementation lacks the last 2 optional parameters from the builtin Map functionality
• by not using a for loop and storing the length, is not required to fetch the this.length value for each iteration
• it doesn't pollute the global space (lodash export the functions, and you use them as you need), and most important it doesn't "modify" all instances of arrays from the entire project, I mean for many reasons it is bad to do Array.prototype.ownMap