[Waterluvian]

I think it should be explicit without a default.

But in the real world, if we must have a default, I think it should be 1, not infinity.

Infinity has a frustrating fail state, in my opinion: you might be flattening arrays that weren't meant to be flattened. And now you've got a cluster!@#$ of data to look at and reason about.

The fail state of 1 is that it doesn't flatten enough but you have the original structure in your 1-level flattened output. So you're far more likely to make heads and tails of how much more flattening you need.

[mstade]

Arguably the same argument works the other way around as well, that in thinking flat recursively flattens all arrays of arrays you get confused when it doesn't. (I've definitely run across that footgun.)

For me, I just find that in almost all cases I've used flat it's been with Infinity as the depth value. It seems to me as well that most of the time you'd probably either want 1 or Infinity, not 5 or 12 or whatever, so perhaps it would've even been better to just have two functions: flat and flatDeep (or some such, naming is hard) the latter of which would default to Infinity but allow different depths as well.

Probably reads better than just flat as well, e.g. `.flatDeep(5)` rather than `.flat(5)`. Oh well, we're stuck with this now so it's all an academic exercise anyway, but I'd be curious to know the rationale of the design. Maybe I'll wade through the spec repo one day to see if there's any discussion about it.