On Sat, Oct 24, 2015 at 8:00 PM, Sebastian McKenzie <[email protected]>
wrote:
> I was recently reading the specification on the behaviour of duplicate
> super() calls in derived constructors. Reading the grammar the following is
> valid:
>
> ```
> class Foo {
> constructor() {
> console.log("foobar);
> }
> }
>
> class Bar extends Foo {
> constructor() {
> super();
> super();
> }
> }
>
> new Bar;
> ```
>
> However my reading of the runtime semantics, it will actually execute the
> constructor body *twice. *And so "foo" will be printed two times.
>
> This is because the only thing I've found stopping duplicate super calls
> is step 10 of SuperCall in 12.3.5.1
> <http://www.ecma-international.org/ecma-262/6.0/index.html#sec-super-keyword-runtime-semantics-evaluation>
> :
>
> 1. Let newTarget be GetNewTarget().
> 2. If newTarget is undefined, throw a ReferenceError exception.
> 3. Let func be GetSuperConstructor().
> 4. ReturnIfAbrupt(func).
> 5. Let argList be ArgumentListEvaluation of Arguments.
> 6. ReturnIfAbrupt(argList).
> 7. Let result be Construct(func, argList, newTarget).
> 8. ReturnIfAbrupt(result).
> 9. Let thisER be GetThisEnvironment( ).
> 10. Return thisER.BindThisValue(result).
>
> Since BindThisValue will throw a ReferenceError if `this` has already been
> initialised:
>
>
> 1. Let envRec be the function Environment Record for which the method
> was invoked.
> 2. Assert: envRec.[[thisBindingStatus]] is not "lexical".
> 3. If envRec.[[thisBindingStatus]] is "initialized", throw a
> ReferenceError exception.
>
> But this check is performed at step 10 whereas the super constructor is
> actually evaluated at step 7.
>
> Is my reading correct? If so, to me this behaviour seems quite unexpected.
>
I'm surprised. Good catch! Frankly, this case had never occurred to me and
I do not remember discussing it. I would certainly prefer that it be an
error without constructing twice.
>
> Any insight (or correction) is extremely appreciated, thank you!
>
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>
>
--
Cheers,
--MarkM
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