d at domenic.me (2015-01-28T19:34:59.774Z)
Maybe this is wrong, but I'm in the habit of thinking that if something has a value of null, something in JavaScript code (mine or a library I'm using) explicitly set it to null. If something has a value of undefined, it was never set. For that reason, I'd prefer using undefined in this case over null.
Maybe this is wrong, but I'm in the habit of thinking that if something has a value of null, something in JavaScript code (mine or a library I'm using) explicitly set it to null. If something has a value of undefined, it was never set. For that reason, I'd prefer using undefined in this case over null. __ __ / \/ \ \ /ark Object Computing, Inc. \ / \/olkmann On Mon, Jan 19, 2015 at 10:54 AM, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote: > > On Jan 19, 2015, at 5:51 AM, Claude Pache wrote: > > > Le 19 janv. 2015 à 11:58, Andreas Rossberg <rossberg at google.com> a écrit : > > On 17 January 2015 at 19:14, Allen Wirfs-Brock <allen at wirfs-brock.com> > wrote: > >> On Jan 17, 2015, at 9:53 AM, Domenic Denicola wrote: >> > On Jan 17, 2015, at 12:31, Allen Wirfs-Brock <allen at wirfs-brock.com> >> 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 > > > _______________________________________________ > es-discuss mailing list > es-discuss at mozilla.org > https://mail.mozilla.org/listinfo/es-discuss > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.mozilla.org/pipermail/es-discuss/attachments/20150119/deb97ca7/attachment.html>