Strawman proposal: new `is` operator
brendan at mozilla.org
Mon Aug 25 18:56:51 PDT 2014
Isiah Meadows wrote:
> Cc the list...
> On Aug 25, 2014 6:06 PM, "Isiah Meadows" <impinball at gmail.com
> <mailto:impinball at gmail.com>> wrote:
> There really shouldn't be any sort of object construction needed
> to check types like this. `isa` may be better, anyways, but I
> still find that requirement to build and destroy an object to
> check somewhat counterproductive.
What object is built and destroyed?
Note that for primitive types no wrapper need be created just to call a
method, in general for JIT-optimized code, and definitely for strict
> Andrea gets what I'm talking about. Also, another (possibly
> separate) proposal would be to make cases like `"foo" instanceof
> String" === true` instead of their current behavior, throwing a
> TypeError. (I believe...I'm on a phone, not a PC where I can test.
> Correct me if I'm wrong.)
(Your phone doesn't have a browser with a console? :-P)
js> "hi" instanceof String
js> 42 instanceof Number
js> false instanceof Boolean
These are well-defined for instanceof, without throwing. Changing
results to true would be backward incompatible.
ES4 had 'is' as a type-classifying operator:
It would be a mistake to define 'is' without defining the (unsound) type
system it depends on. This is a challenge, but TypeScript and other
close-to-ES6 languages have sallied forth. We need a detailed proposal.
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