Assignment to wrapper objects in strict mode

Andreas Rossberg rossberg at
Wed May 28 05:33:50 PDT 2014

On 28 May 2014 14:08, André Bargull <andre.bargull at> wrote:
> On 5/28/2014 2:00 PM, Andreas Rossberg wrote:
>> On 28 May 2014 13:41, André Bargull <andre.bargull at> wrote:
>>>> Apparently, there has been a semantic change between ES5 and ES6
>>>> regarding assignment to wrapper objects in strict mode. That is,
>>>>     'use strict'; "".x = 0
>>>> would throw in ES5, but AFAICS, no longer does in ES6. Was this change
>>>> discussed? What is the rationale?
>>>> (FWIW, current implementations disagree about this: e.g., V8 and SM
>>>> don't throw, JSC does.)
>>> That assignment still throws per the current ES6 draft. Following
>>> PutValue to 9.1.9 [[Set]], step 4.d in [[Set]] creates a dummy data
>>> descriptor when no inherited property was found (that way the condition
>>> in
>>> step 5 will evaluate to true), and step 5.b in [[Set]] returns false if
>>> the
>>> receiver is not an object. At that point we're back in PutValue, and step
>>> 6.d in PutValue will now throw TypeError in strict mode.
>> But base will already have been converted to a wrapper object at
>> PutValue step 6.a.ii before you get to [[Set]].
> The receiver argument in [[Set]] is not the wrapped `base`, but
> `GetThisValue(V)`, here: the empty string "".

Ah, you are right, thanks. Sorry for the noise.


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