W.r.t auto-boxing "this" values, I just found out that this is one of the
behaviors that "use strict" changes. Behold:
Number.prototype.normal = function(){return typeof this;};
Number.prototype.strict = function(){"use strict";return typeof this;};
(1).normal(); // returns "object"
(1).strict(); // returns "number"
Yay javascript?
On Tuesday, December 17, 2013 11:12:55 AM UTC-6, Chas Emerick wrote:
> Yeah, I've grokked it now. I did not appreciate the subtleties of
>
> boxing w.r.t. typeof. My intuition was that a boxed numeric was
>
> nevertheless a "number"; the fact that strings are apparently themselves
>
> boxed and yield a typeof of "object" confused things further for me (I'd
>
> never think of strings as primitives...certainly, they're not, at least
>
> not in the way numbers can be).
>
>
>
> Anyway, so carpet-bombing the methods I'm installing on JS native
>
> prototypes with `(.valueOf this)` is the real fix, though it seems
>
> error-prone and wasteful since this treatment should probably be applied
>
> anywhere a value escapes from a prototype method.
>
>
>
> Rhetorically, I'd wonder:
>
>
>
> 1. If there's something ClojureScript should be doing w.r.t. protocol
>
> dispatch to make this less thorny, though I'm aware that any "solution"
>
> would probably carry a performance penalty…
>
> 2. If this might not be an ideal context for some static analysis to
>
> warn when a boxed value may be headed into a protocol method invocation…
>
>
>
> Thanks all for the hints and help,
>
>
>
> - Chas
>
>
>
> On 12/17/2013 10:45 AM, Francis Avila wrote:
>
> > The behavior you are seeing is due to javascript auto-boxing the primitive
> > types. number, string, etc are not objects and have no properties to call.
> > When the js runtime sees you look up a "property" of a non-object type it
> > boxes the value with the appropriate object (Number for number, String for
> > string, etc) before doing protocol lookup. The "this" value you are seeing
> > is the box, which is why its typeof is object.
>
> >
>
> > You can retrieve the unboxed value from the box using this.valueOf()
>
> >
>
> > On Tuesday, December 17, 2013 7:57:12 AM UTC-6, Chas Emerick wrote:
>
> >> As much as I distrust goog.typeOf, this isn't its fault. It's really
>
> >>
>
> >> quite bizarre: AFAICT, typeof (i.e. the JavaScript operator, not the
>
> >>
>
> >> goog.typeOf function) returns "object" for any value named by `this`:
>
> >>
>
> >>
>
> >>
>
> >>> typeof 5
>
> >> "number"
>
> >>
>
> >>> Number.prototype.foo = function () { return typeof this; };
>
> >> [object Function]
>
> >>
>
> >>> (5).foo()
>
> >> "object"
>
> >>
>
> >>
>
> >>
>
> >> This is a whole new level of JavaScript wat for me. I've asked the SO
>
> >>
>
> >> hordes for insight:
>
> >>
>
> >> http://stackoverflow.com/questions/20636028/typeof-returns-object-for-this-number-elsewhere
>
> >>
>
> >>
>
> >>
>
> >> Meanwhile, I've discovered that extending the protocol to both `number`
>
> >>
>
> >> and `js/Number` does work under both execution contexts, a much more
>
> >>
>
> >> palatable workaround. That's allowed me to move forward with my
>
> >>
>
> >> current work.
>
> >>
>
> >>
>
> >>
>
> >> Thanks,
>
> >>
>
> >>
>
> >>
>
> >> - Chas
>
> >>
>
> >>
>
> >>
>
> >> On Tue 17 Dec 2013 04:37:56 AM EST, Thomas Heller wrote:
>
> >>
>
> >>> FWIW I had some troubles with native types before, mine were related to
> >>> IE though. The culprit for your troubles is goog.typeOf (which is a basic
> >>> wrapper arrount js/typeof).
>
> >>> If you run this via phantomjs
>
> >>> -----
>
> >>> console.log("typeof", typeof(5));
>
> >>> Number.prototype.foo = function() {
>
> >>> console.log("typeof prototype", typeof(this));
>
> >>> };
>
> >>> (5).foo();
>
> >>> phantom.exit(0);
>
> >>> -----
>
> >>> You get
>
> >>> typeof number
>
> >>> typeof prototype object
>
> >>> Pretty sure the problem goes away cause in advanced compilation cause it
> >>> gets inlined and (.foo 5) is turned into (m 5) skipping the step with
> >>> this. Just a guess though.
>
> >>> HTH,
>
> >>> /thomas
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