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Comment by dmitriid | Hacker News Reader

I wonder if there are enough use-cases for .flat to not default to Infinity.

It's also confusing that `arr.flatMap()` is not equivalent to `arr.map().flat()`, but to `arr.map.flat(Infinity)`

According to MDN `arr.flatMap()` is indeed equivalent to `arr.map().flat()` (without the Infinity). [1] Testing in the Chrome Devtools it also seems to be the case:

  x = [[[1, 2]], [[2, 3]], [[3, 4]]]
  x.flatMap(x=>x)
  output: [[1,2], [2,3], [3,4]]
  x.map(x=>x).flat()
  output: [[1,2], [2,3], [3,4]]
  x.map(x=>x).flat(Infinity)
  output: [1, 2, 2, 3, 3, 4]

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe... > It is identical to a map() followed by a flat() of depth 1, but flatMap() is often quite useful, as merging both into one method is slightly more efficient

Is there any reason it isn't called mapFlat? FlatMap indicates to me that there's a flattening, then a mapping, not the other way around.

Ah, I missed that. I though flatMap had depth of Infinity.

Infinity was originally the default (which feels intuitive), but here's the argument for changing it [0]:

"I think Array#flatten should be shallow by default because it makes sense to do less work by default, it aligns with existing APIs like the DOM Node#cloneNode which is shallow by default, and it would align with the existing ES3 pattern of using Array#concat for a shallow flatten. Shallow by default would also align with flatMap too."

[0] https://github.com/tc39/proposal-flatMap/issues/9

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