Implement Map function using Reduce in TypeScript

Question

I need to implement the mapping function using reduce.

Requirements:

The map function should apply a fn function to every element in array.
array should not be mutated

This is my code so far (I'm checking if the array is empty):

function map<A, B>(array: A[], fn: (a: A) => B): B[] {
  if (array.length === 0) {
    return [];
  } 

}

Would appreciate any help!

@phuzi that's the task - using map is forbidden. I need to impement it with reduce :) — Anna Harshyna
– Anna Harshyna, Commented Apr 23, 2021 at 7:21
the check is not necessary, if the array is empty it returns empty anyways — Leigh Cheri
– Leigh Cheri, Commented Apr 23, 2021 at 7:26

Evan Trimboli · Accepted Answer · 2021-04-23 07:41:15Z

1

You could do something like this (untested):

const map = <A, B>(array: readonly A[], fn: (a: A) => B): B[] => {
  return array.reduce((all, item) => {
    all.push(fn(item));
    return all;
  }, [] as B[]);
};

answered Apr 23, 2021 at 7:41

Evan Trimboli

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8 Comments

captain-yossarian from Ukraine Over a year ago

@AnnaHarshyna please use return [...acc, fn(item)]. Avoid mutation inside reduce

Evan Trimboli Over a year ago

@captain-yossarian Care to explain? In this particular circumstance, what benefit is there?

captain-yossarian from Ukraine Over a year ago

Reduce is more functional approach. It can be used for function composition. In general, nobody expects mutation here. Btw, I believe it is better to avoid mutation of arguments. But it is only my thoughts. I respect your approach.

Evan Trimboli Over a year ago

Nobody expects mutation of a variable local to only this function that the function itself creates? I don't understand why that would be problematic.

captain-yossarian from Ukraine Over a year ago

Are You open for explanation and arguments or You just want to argue? I'm asking, because if You are FP hater , nobody wins in this discussion

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