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Comment by marcta

10 hours ago

Why is arr.filter().map() better than arr.reduce()? Doesn't arr.reduce() only loop once through the array?

4 comments

marcta

Reply
ema  9 hours ago

For me it's about the speed of understanding what the code does. Because filter and map are very constrained in what they can do they quickly tell me a lot about the shape of the computation I'm working with. In contrast reduce is much more flexible and so I need to do a much more detailed analysis to just answer a basic question like "is the result a scalar or another collection?".

FlameWolf  10 hours ago

Exactly. If I need just filter/map/some etc., I use it. But if I need a combination of more than one, that's a job for reduce().

  • ggorlen  3 hours ago

    So you prefer

        arr.reduce((acc, el) => {
      if (el % 2 === 0) {
        acc.push(el * 2);
      }

      return acc;
    }, []);

    over

        arr.filter(e => e % 2 === 0).map(e => e * 2)

    The only advantage of the reduction as I see it is performance, but this is highly dubious and would need to be profiled for proof (I don't recall seeing removing a pass like this matter in practice). And if perf does matter, a for..of loop would be clearer and one-pass, not to mention async-compatible:

        const result = [];
    for (const el of arr) {
      if (el % 2 === 0) {
        result.push(el * 2);
      }
    }

    Exercises like this illustrate why verbal technical job interviews are useful in the age of LLMs--a series of A/B taste preferences seems high signal and ripe for discussion: "Ah, so you're choosing reduce for perf... please describe a scenario you encountered where this made a measurable impact".

QuercusMax  10 hours ago

Chained filter and map don't necessarily iterate multiple times. They certainly can but depending on how things are built they very often run as a single loop with the operations chained.