Destructuring and evaluation order

Allen Wirfs-Brock allen at
Fri Apr 25 10:42:53 PDT 2014

On Apr 25, 2014, at 10:25 AM, Oliver Hunt wrote:

> On Apr 25, 2014, at 4:24 AM, Andreas Rossberg <rossberg at> wrote:
>> The way destructuring assignment currently is specified leads to
>> rather inconsistent evaluation order. Consider:
>> let o = {}
>> function f() { print(1); return o }
>> function g() { print(2); return 5 }
>> f().x = g()    // 1, 2
>> {a: f().x} = {a: g(}}   // 2, 1
>> That is, destructuring assignments violate the left-to-right
>> evaluation that is otherwise maintained.
> I don’t believe that there is any reason to break from left-to-right evaluation order.  Even if it was uncommon it’s still an unnecessary hazard.
> Spec bug maybe?

Nope, it's always been designed this, going back to the original wiki strawman and I assume the original FF implementation.

It has also been something that we has been discussed here and in TC39 multiple times.  If we wan to revisit this decision we really need to dredge up all of those previous discussions to see if there is new data that would cause us to change this decision. 

Basically for destructing assignment, each property element can be an arbitrary assignment expression and all of their Reference values would have to be saved to get left to right order. 


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