[Webkit-unassigned] [Bug 31813] Add support for block scope const

Wed Nov 16 06:48:26 PST 2011

https://bugs.webkit.org/show_bug.cgi?id=31813

Andy Wingo <wingo at igalia.com> changed:

           What    |Removed                     |Added
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                 CC|                            |wingo at igalia.com

--- Comment #1 from Andy Wingo <wingo at igalia.com>  2011-11-16 06:48:26 PST ---
I would like to implement this, so if you don't mind I will use this bug for stating the requirements, coming to a strategy, and fleshing out an implementation.

To summarize, there are currently two ES modes:

* Strict
* "Classic" (non-strict)

ES.next adds a third:

* "Harmony"

In classic mode, `const' has no specification.  In JSC it is like `var', and hoisted to function scope.

In strict mode, `const' is a syntax error.

In harmony mode, `const' is block-scoped, with a so-called "temporal dead zone": when the var is first made, it is logically populated with some sentinel value.  If the var is accessed before it is set, that is a runtime error.  If it is set twice, that is a runtime error.  In some cases the compiler can elide the temporal dead zone, but not in general.

There are also some wrinkles that in top-level strict contexts, `const' should create non-writable, non-configurable properties.  Full details on the linked page.

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