On Jan 19, 2015, at 5:51 AM, Claude Pache wrote: > >> Le 19 janv. 2015 à 11:58, Andreas Rossberg <[email protected]> a écrit : >> >> On 17 January 2015 at 19:14, Allen Wirfs-Brock <[email protected]> wrote: >> On Jan 17, 2015, at 9:53 AM, Domenic Denicola wrote: >> > On Jan 17, 2015, at 12:31, Allen Wirfs-Brock <[email protected]> wrote: >> >> >> >> If the enclosing function is invoked as a call expression the value of >> >> `new.target` is null >> > >> > Just curious, why null instead of undefined? >> >> null is used to indicate no [[Prototype]], so it seem to me to be a better >> match for this situation. >> >> Wouldn't the fact that null is a quasi-legal prototype strongly speak for >> using undefined here? Otherwise, it seems you couldn't distinguish Call >> invocations from Construct invocations with a prototype that has actually >> been set to null (which I suppose is legal?). >> >> (In terms of proper option/maybe types, this is yet another case of a None >> vs Some(None) distinction.) >> >> /Andreas >> > > `new.target` is a reference to the constructor, not the prototype, so the > problem does not arise in practice. > > But anyhow, I do think that `undefined` is semantically better here: > > * `new.target === null` means: `new.target` has been set to "no object". > * `new.target === undefined` means: `new.target` has not been set.
At the JS level, I don't actually think about `new.target` as something that is "settable". I think about it as an oracle that tells me about how this function was invoked. null means it was invoked "as a function". non-null means it was invoked as a constructor and the value is the object that `new` was applied to. > > When you execute a function body with the semantics of [[Construct]], the > value of `new.target` is the original constructor on which `new` was applied. > If it was possible to have the semantics of [[Construct]] with no original > constructor, then `new.target` would be `null` (no-object). But it isn't. Reflect.construct ensures that an non-null value is passed to [[Construct]] as its second argument > > But when you execute a function body with the semantics of [[Call]], there is > no notion of "original constructor", and `new.target` is left with no value, > i.e. `undefined`. I originally intended `new.target` to use `undefined` is the sentinel value to indicated "called as a function". But as I wrote the spec. it felt better to use `null` in that role. I think it's because using `null` seems more special. `undefined` is used in so many places to indicated so many different things that it is hard to apply any generalized meaning to it. On the other hand, `null` is used in only a few places in the ES spec. so its use seems to draw attention to the specialness of those situations. But, I'd have no problem with changing back to `undefined` if there is a consensus in favor of that. It really makes very little difference as `null` and `undefined` are both falsey values, so the preferred way to write a "called as a function" these should probably be: `if (! new.target) ...` Allen
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