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Understanding Array reduce by building it from scratch

Ben Lesh on December 31, 2019

Recently, I was involved in a thread on Twitter, where I mentioned that I also, at one time, found Array reduce challenging to wrap my head around....

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Lester Dale • Dec 31 '19

I feel I have the potential to be an excellent developer but am not sure what steps to take next to become a Jedi, right now the force is weak with me. Can I get some references to good collaborative open source development I would appreciate this kind people.

Keshav Kaul • Jan 1 '20

Thanks for this simple article on reduce! I have a suggestion for changing the map method to a more performant version. Would love to see an article on rxjs's scan method!

The performance of this map is n^2 because we are returning a new copied array inside the reducer method. Isn't this unnecessary immutability? Because we control the initial empty array state, we could mutate this array inside the reducer and still benefit from mutability for the caller of the map method.

Instead of this

Array.prototype.map = function (callback) {
  return this.reduce(
    (result, item, i, source) =>
      [...result, callback(item, i, source)],
    []
  );
}

We could do this -

Array.prototype.map = function (callback) {
  return this.reduce(
    (result, item, i, source) =>
      {
        result.push(callback(item, i, source))
        return result
      },
    []
  );
}