README.md

Separate odd and even numbers

The problem

You have a list of integers:

const integers = [1, 42, 7, 37, 2, ...]

Your goal is to separate odd and even numbers:

const separated = {
  odd: [1, 7, 37, ...],
  odd: [42, 2, ...],
}

Slow solution

function slow_solution(integers) {
  return integers.reduce(
    ({ odd, even }, n) => {
      if (n % 2 === 0) return { odd, even: [...even, n] };
      return { odd: [...odd, n], even };
    },
    { odd: [], even: [] }
  );
}

Why is this slow?

We are creating a new array and new object on each iteration of the reduce callback.

Faster reduce solution

function faster_reduce_solution(integers) {
  return integers.reduce(
    (res, n) => {
      if (n % 2 === 0) res.even.push(n);
      else res.odd.push(n);
      return res;
    },
    { odd: [], even: [] }
  );
}

Why is that faster?

This mutates the arrays instead of creating new ones. So we avoided creating unneeded objects successfully.

Alternative solution without mutation

if your coding style forces you not to mutate variables. You can avoid using .reduce and use .filter instead. Which is cleaner in my opinion:

function faster_filter_solution(integers) {
  return {
    odd: integers.filter((n) => n % 2 === 1),
    even: integers.filter((n) => n % 2 === 0),
  };
}

Benchmark

	slow_solution	faster_reduce_solution	faster_filter_solution
1k integers	5.216 ms	0.247 ms	0.215 ms
10k integers	57.073 ms	0.541 ms	0.638 ms
100k integers	27299.561 ms	3.823 ms	2.969 ms

Benchmark run on Github actions