On May 10, 2012, at 1:08 AM, Axel Rauschmayer wrote: >>> Are we going to have RegExp.isRegExp() and Date.isDate() and >>> Number.isNumber() etc. too ? >> >> I did wince a bit about ES5's Array.isArray -- perhaps since it takes any x >> and tells whether x is an Array (from any window or frame) instance, it >> should have gone on object. You're right that the instanceof limitation >> motivating Array.isArray applies to these and other built-in "classes" as >> well. >> >> Do we just add 'em all, or try to add only the ones for which we have enough >> experience to hope we're actually helping developers? How many times have >> you wanted RegExp.isRegExp, say? The belief is that testing for array-ness >> is more common. I don't have data, though, so I'm uncomfortable with any >> "need-based" approach. >> >> Adding lots of predicates is straightforward, but then the dialectic >> requires us to wince at the sheer number of predicates (and the redundant >> name stems), and try for something like what Axel suggested: a do-it-all >> "type" or "typeOf" or "classify" API. >> >> Comments welcome. I'm not sure what is best but lean toward predicates as >> goods in themselves, even if not the complete solution (if there is a >> complete solution). > > It would be great if we could eliminate these predicates completely. How > often does the frame crossing problem matter in practice? It doesn’t show up > in non-browser environments, right? > > I see several possibilities: > - Make instanceof work correctly with objects that have crossed frames. > - Introduce a binary predicate, e.g. likeInstanceOf that correctly handles > cases where the lhs and rhs come from different contexts/frames. > > Additionally, one could throw an exception if there is an instanceof check > whose lhs and rhs are from different contexts (failing fast, preventing > obscure and hard-to-debug errors).
When you want to dispatch based on a type, a fixed typeof (one that worked well) would be better: switch (typeof x) case "Array": case "RegExp": case ... else you'd have to do: if (Array.isArray(x)) ... else if (RegExp.isRegExp(x)) ... else if ( etc ) When you just want to assert that x is of type Type, then an if (Type.isType(x)) would be ok, but a proper typeof would do just as well: if (typeof x === "Type") So it seems that a new, fixed typeof would be best? And if the new, fixed typeof were typeOf(), with capital O, a global function instead of a language keyword/operator? That would be easily polyfill-able. -- Jorge. _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
