More fun with undefined

Aymeric Vitte vitteaymeric at gmail.com
Fri Jun 15 07:34:27 PDT 2012


I am not talking about defining implicit properties or such things, 
neither having undeclared stuff looking declared, but just changing the 
behavior of retrieving a property when base is undefined, which will 
then be undefined.

If am I reading correctly the specs, doing this change will work for 
a.b.c.d, because undefined is returned first and nothing is set 
anywhere, unless I am wrong :

console.log(a); //reference error

var a;//undefined
console.log(a.b);//reference error

will become :

console.log(a); //undefined

var a;//undefined
console.log(a.b);//undefined

Le 15/06/2012 15:52, T.J. Crowder a écrit :
> On 15 June 2012 14:34, Aymeric Vitte <vitteaymeric at gmail.com 
> <mailto:vitteaymeric at gmail.com>> wrote:
>
>     Example :
>
>     console.log(a);//Reference error, GetBase returns undefined
>
>     console.log(window.a);//undefined
>
>     --> does not seem very logical, no ?
>
>
> To me this would be a big step backward, after the very large stride 
> forward this group made in ES's strict mode of making _assigning_ to 
> an unresolvable reference an error rather than an implicit creation of 
> a property on the global object.
>
> For one thing, how is the engine to know that the `a` in question was 
> meant to be `window.a`? Maybe I just forgot to put `var a` in the 
> current scope. (In fact, that's usually what it is when I get this error.)
>
> Scope is not the same as an object (although of course, the scope 
> chain is conceptually made up of binding objects). If I refer to `a` 
> in my code and `a` has never been declared, that's a bug, and as I 
> haven't told it, the engine has no way of knowing what level in the 
> scope chain I intended `a` to be in. If I access the `a` property of 
> an object, and the property has never been defined, that could just be 
> lazy initialization; the engine knows that I'm talking about that 
> specific object (or its prototypes), because I've told it what object 
> to look at.
>
> Separately:
>
>     (ie a way that this works : if (a.b.c.d) {} when a,a.b, etc are
>     not set)
>
>
> Making the initial `a` evaluate to `undefined` wouldn't make that 
> work: Instead of the ReferenceError, you'd get a TypeError (cannot 
> read property `b` of undefined).
>
> -- T.J.

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