Default function arguments not using current scope

[Cyril Auburtin]

Thanks for the clear explanation

2018-04-05 11:08 GMT+02:00 T.J. Crowder <tj.crowder at farsightsoftware.com>:

> On Thu, Apr 5, 2018 at 9:29 AM, Cyril
> Auburtin <cyril.auburtin at gmail.com> wrote:
> >
> > I don't understand:
> >
> > ```js
> >
> > var z=1; var fn = (z = z) => z; fn()
> > // throws ReferenceError: z is not defined
> > ```
>
> SpiderMonkey's error message is a bit clearer here:
>
> > ReferenceError: can't access lexical declaration `z' before
> initialization
>
> Remember that default parameter values have their own scope. In that
> scope, they have a TDZ (like `let` and `const` do in a block) until they're
> initialized. I believe this is covered in [FunctionDeclarationInstantiation][1]
> Step 28. At the point the default value expression `= z` is evaluated, the
> `z` parameter has been created as an uniniitialized binding (by
> [CreateMutableBinding][2] in 28.f.i.2). Consequently, it tries to get the
> value of that uninitialized binding, which results in a ReferenceError as
> described in Table 14 under [Environment Records][3]:
>
> > > GetBindingValue(*N*, *S*) - Returns the value of an already existing
> binding from an Environment Record. The String value *N* is the text of the
> bound name. *S* is used to identify references originating in strict mode
> code or that otherwise require strict mode reference semantics. If *S* is
> true and the binding does not exist throw a ReferenceError exception. **If
> the binding exists but is uninitialized a ReferenceError is thrown**,
> regardless of the value of *S*.
>
> *(My emphasis.)*
>
> E.g., you get that error for the reason you get that same error from:
>
> ```js
> var z = 42; { console.log(z); let z; }
> ```
>
> -- T.J. Crowder
>
> [1]: https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation
> [2]: https://tc39.github.io/ecma262/#sec-declarative-environment-records-
> createmutablebinding-n-d
> [3]: https://tc39.github.io/ecma262/#sec-environment-records
>
>
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