issue: function hoisting and parameter default value initialization

Allen Wirfs-Brock allen at wirfs-brock.com
Mon Oct 8 10:18:49 PDT 2012


On Oct 8, 2012, at 10:04 AM, Andreas Rossberg wrote:

> On 8 October 2012 18:24, Allen Wirfs-Brock <allen at wirfs-brock.com> wrote:
>> You desugaring would break ES<=5.1 compatibility for:
>> 
>> (function (p) {
>>   var p;
>>   return typeof p
>> })("test")
>> 
>> ES5.1 returns "string", your desugaring returns "undefined"
> 
> I'm probably just being blind, but I'm not seeing it. Why?
> 
> /Andreas
> 
 In ES <= 5.1, function declarations over-write formal parameters.  But var declarations, without initializer do not assign undefined to already local bindings (including formal parameters) that already exist.

You desugaring would create a new var declaration in a new scope contours that would be initialized to undefined and shadow the like-named formal parameter.

Allen




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