Strict mode and early errors

Mark S. Miller erights at
Wed Oct 13 08:28:22 PDT 2010

It is a bug in WebKit. Consider

  (function(global) {
    'use strict';
    global['foo'] = 88;
    foo = 99;

This code is not erroneous. By the time it assigns to global foo, there is a
global foo and so there's no problem to report. Minefield (FF nightly) does
this correctly. WebKit reports this as a SyntaxError (I just checked), since
"early" is too early to tell in general that there will be nothing wrong by
the time the assignment is executed.

That aside, I'd like to offer my congratulations to WebKit for taking this
huge step towards strict mode support!

On Wed, Oct 13, 2010 at 6:59 AM, Juriy Zaytsev < at> wrote:

> Strict mode support has just landed in WebKit. While running few simple
> tests on it (, I
> noticed that WebKit reports some of the errors early. For example, as far as
> I understand, assignment to an unresolvable reference (in strict mode) ought
> to throw ReferenceError — as explained in 11.13.1 (Simple Assignment) and
> 8.7.2 (PutValue).
> "use strict";
> i_dont_exist = 1; // <-- should throw ReferenceError
> However, in WebKit this is a SyntaxError (with a somewhat cryptic message
> of "Parse Error", but that's irrelevant here).
> What I'm not sure about is whether this is allowed per Chapter 16. The list
> of early errors does not include this scenario. It does allow to treat
> "assignment to non-reference" as an early error (e.g. 3 = 4), but says
> nothing about assignment to an unresolvable reference. It also seems to
> disallow any deviations in a very explicit manner:
>   "An implementation shall not treat other kinds of errors as early errors
> even if the compiler can prove that a construct cannot execute without error
> under any circumstances. An implementation may issue an early warning in
> such a case, but it should not report the error until the relevant construct
> is actually executed."
> The reason I'm bringing this up is because my test suite expected
> ReferenceError, and so looked like:
> try {
>   i_dont_exist = 1;
> }
> catch(err) {
>   assert(err instanceof ReferenceError);
> }
> ...which resulted in SyntaxError, preventing entire set from running.
> Is this kind of early errors allowed and I missed something in the specs?
> Or should one of — WebKit or spec —  be changed accordingly? Note that
> nightly FF — the only other browser with strict support at the moment —
> throws ReferenceError, as expected.
> cc'ing Oliver
> Thank you.
> --
> kangax
> _______________________________________________
> es5-discuss mailing list
> es5-discuss at

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