> 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] > <mailto:[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] > > <mailto:[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. 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 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`. —Claude
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