Negative zero (-0) is the only value you can't use as a Map key or a Set value. Map's [`set`][1] and Set's [`add`][2] both explicitly replace it with +0. MDN makes a point of saying that "early drafts" of the ES2015 spec treated +0 and -0 as distinct, but that "changed" in ES2015. Looking at some early drafts, at some stage Map and Set used SameValue (so you could have both +0 and -0 as Map keys or in a Set), then went through a period where you could specify a "comparator" for them (so a given instance would use SameValue or SameValueZero). Then that was removed; the final spec uses SameValueZero and has `set` and `add` replace -0 with +0.
Using SameValueZero and converting -0 to +0 when storing does have the effect of ensuring consistency: You don't end up with two sets (for instance) with different values (-0 and +0) that Set's comparison logic (SameValueZero) considers the same. (But then, so would using SameValue and converting from -0 to +0 earlier in the `set` and `add` algorithsm.) Using SameValue and allowing -0 would also ensure consistency, but with the ambiguity that -0 and +0 are often hard to distinguish (Number's `toString` returns `"0"` for both, for instance; operations involving them largely don't care about the pseudo-sign of 0). Is that the rationale? Basically, avoid a footgun while ensuring consistency? Thanks, -- T.J. Crowder [1]: https://tc39.github.io/ecma262/#sec-map.prototype.set [2]: https://tc39.github.io/ecma262/#sec-set.prototype.add
_______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
