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Comment by guiomie | Hacker News Reader

In his example Array.prototype.reduce, isn't this almost the same as just doing a foreach loop? I don't understand why doing this, it's pretty much the same number of lines.

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In this example it is, but a nice thing about using map, reduce etc is that you can chain them together, ex:

  let out = ["a", "b", "c", 1, 2, 3]
  	// Turn letters to upper case
  	.map(i => {
  		return typeof i === "string"
  			? i.toUpperCase()
  			: i;
  	})
  	// Add one to numbers
  	.map(i => {
  		return typeof i === "number"
  			? i + 1
  			: i;
  	})
  	// split letters and numbers up
  	.reduce((all, i) => { 
  		typeof i === "string"
  			? all.letters.push(i)
  			: all.numbers.push(i);
  		return all;
  	}, { "letters": [], "numbers": [] });
  // => { letters: ["A", "B", "C"], numbers: [2, 3, 4] }

Or... you can do all of that in a 4-line loop, without any of the harm to readability or additional code-bloat that's especially important for websites.

The problem with this is that it generates two intermediate arrays in the maps, then effectively drops them in the reduce. This may be inefficient.

I wish that JavaScript's map/filter/reduce functions returned lazy iterators, like in Rust, so that code like this doesn't produce intermediate arrays. Does anyone know of a library that provides this?

Those intermediate iterations could be dropped by composing the map functions together (same with the reduce), if that really was a performance bottleneck.

With lazy iterators, there is the possibility of better composition of maps/filters/reduces. That's why I'm a fan of the lazy iterators approach -- otherwise the abstraction breaks too easily and it ends up looking like a for-loop anyway.

That's the thing about functional programming. I personally don't remember the last time I had the need to write a foreach loop. Foreach loops even kind of feel _dirty_ to me.

All I really need now is Array.map, Array.reduce, Array.filter, Array.sort, Object.keys, Object.values.

Reduce is very powerful, you just have to get to know it.

Edit: this is how I'd use reduce in this case — https://news.ycombinator.com/item?id=14919636 — talk about line numbers

> isn't this almost the same as just doing a foreach loop

yes they are both ways of iterating over the array

> I don't understand why doing this

there's more than one way to skin a cat

foreach implies mutation. reduce is a way to aggregate a collection into single result in an immutable way.

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