Set the array length explicitly

[ljharb]

Fixes #41

[michaelficarra]

This adds an observable Set operation for each recursive call of FlattenIntoArray. I don't think that's what we want. We only want one Set operation on the length property after the work has been completed.

[ljharb]

That'd be ideal; but 1) that's not how any of the existing examples in the spec do it, 2) if the TypeError exception gets thrown, the length wouldn't get set at all, 3) array subclasses might do a bunch of weird stuff when the length is set, and that'd be expected to happen when each item is added.

It also wouldn't be observable on an unmodified Array.prototype with a native array being the target, which I assume is how implementations optimize all the other spec operations.

[michaelficarra]

@ljharb That's not how, for example, Array.prototype.push works. It only sets the length once. Also, it's not just an unmodified Array.prototype, but also Object.prototype.

[ljharb]

Agreed on the unmodified prototypes for sure.

That's a fair point, given that, I'll set it once at the end instead.

[ljharb]

Should the Set be outside of FlattenIntoArray then, since it's recursive? iow, repeated in both flatten and flatMap?

[michaelficarra]

Yeah, I think so.

[ljharb]

Updated.