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Discussion on: Is reduce() bad?

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aendra profile image
Ændra Rininsland • • Edited

There's been so much .reduce() bashing on Twitter in recent days, it's really annoying given how useful and powerful a method it is.

For instance, you can rate limit a bunch of fetch() requests using async/await and .reduce() in a one-liner like so:

(async () => {
  const listOfUrls = ['https://some-api/endpoint/1', 'https://some-api/endpoint/2', 'https://some-api/endpoint/3'];

  const data = listOfUrls.reduce(
     async (acc, cur) => [...await acc, await fetch(cur).then(r => r.json())], Promise.resolve([]));
})();
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(I think; it's been awhile since I wrote one of those)

Granted, with async iterators you can do the following too now:

const listOfUrls = ['https://some-api/endpoint/1', 'https://some-api/endpoint/2', 'https://some-api/endpoint/3'];
const data = [];
(async () => {
  for (const url in listOfUrls) {
    data.push(await fetch(url).then(r => r.json()));
  }
})();
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...Or even:

const listOfUrls = ['https://some-api/endpoint/1', 'https://some-api/endpoint/2', 'https://some-api/endpoint/3'];
const data = [];
(async () => {
  const requests = listOfUrls.map(url => fetch(url).then(r => r.json());
  for await (const datum in requests) {
    data.push(datum);
  }
})();
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...But those are longer, not much more readable, invoke newer ES functionality that other people in your team might be not familiar with and use mutative array methods.

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kenbellows profile image
Ken Bellows •

I don't know, IMO the for loop versions you wrote are far more readable than the reduce version. I typically find that to be true of for vs reduce, which is I think the point of these arguments.

(Unimportant pedantic nitpick: I think you meant to write for ... of rather than for ... in, since in loops over keys rather than values.)

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aendra profile image
Ændra Rininsland •

That's fair! TBTH I'm sort of coming back around on using for loops over .reduce(), async iterators are really nice to use for request throttling and flow control, last project I used them in I was a bit taken aback by how intuitive they felt to use once I got into it (I've never been a huge fan of generators but I'm starting to see their value more now after that).

And yes, I definitely meant for ... of — I get that wrong so often! 😅

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kenbellows profile image
Ken Bellows •

And yes, I definitely meant for ... of — I get that wrong so often! 😅

Ha, same! Especially if I've been writing Python recently, since Python's loop syntax is for x in arr: ...