[throwitaway1123]

It's a little bit faster (on my machine at least) if you combine the filter and map into a flatMap (it's still not as performant as the imperative solution though).

  function process3(input) {
    return input
      .flatMap((n) => (n % 2 === 0 ? n * 2 : []))
      .reduce((a, b) => a + b, 0)
  }

[6510]

If it needs to be fast use oldskool for() loops.

  function processfor(input){
    let sum = 0
    for (let i = 0; i < input.length; i++){
      if (input[i] % 2 !== 0){ continue }
      sum += input[i] * 2
    }
    return sum
  }

https://jsfiddle.net/gaby_de_wilde/y7a39r15/5/

[throwitaway1123]

Yeah the comment I was originally responding to included a for loop (in the Pastebin link). My point is that if you're set on going down the functional route you don't need separate map and filter steps, you can just use flatMap which is effectively a filter and map combined (returning an empty array filters out the current value, since an empty array gets flattened into nothing).

Of course, if you want the most performant solution an imperative for loop is faster (which is what I said in my last comment).

[6510]

input.reduce((a, b) => b % 2 !== 0 ? a : a + (b * 2), 0)

or if you want it really silly.

input.reduce((a, b) => a + (b % 2 && b * 2), 0)

dont ask me why but for(a of b) is slower than for(i=0;i<b.length;i++)

[throwitaway1123]

Array.prototype.reduce can basically do almost anything a for loop can do, since it gives you access to state from one iteration to the next. The only reason I didn't remove the flatMap in my original example and convert it all to reduce, is because there's no longer any method chaining which was the point of the original comparison between the Go and JS examples.

> dont ask me why but for(a of b) is slower than for(i=0;i<b.length;i++)

Probably because for of loops use the iterator protocol. So I'm assuming under the hood the JS engine is actually invoking the Symbol.iterator method (which is slower).

  #! /usr/bin/env node --experimental-strip-types
  
  function processIterator(input: number[]) {
    let sum = 0
    for (let i = input[Symbol.iterator](), r; (r = i.next()); ) {
      if (r.done) return sum
      if (r.value % 2 === 0) sum += r.value * 2
    }
  }
  
  function processfor(input: number[]) {
    let sum = 0
    for (let i = 0; i < input.length; i++) {
      const value = input[i]
      if (value % 2 === 0) sum += value * 2
    }
    return sum
  }
  
  
  const input = Array.from({ length: 1_000_000 }, (_, i) => i)
  
  console.time('normal for loop')
  console.log(processfor(input))
  console.timeEnd('normal for loop')
  
  console.time('iterator for loop')
  console.log(processIterator(input))
  console.timeEnd('iterator for loop')