Bug 182414 – Async arrow function with var declaration has incorrect shadowing

• Component: JavaScriptCore
• Status: RESOLVED DUPLICATE

[Adam Klein]

The following code, run in the shell, should print "7", but prints "undefined":

  (async (v) => { print(v); var v; })(7)

Without "async", this gets the right answer.

[GSkachkov]

Hmm, the same for common async function
```
(async function (v) { print(v); var v; })(7);
```

[GSkachkov]

Yeah and the same for generator and async generators

[Yusuke Suzuki]

Hm, shoud it print 7? Why doesn’t “var” hide?

[GSkachkov]

(In reply to Yusuke Suzuki from comment #3)
> Hm, shoud it print 7? Why doesn’t “var” hide?

I cann't find this rule in current spec but it was in ES5, there is a exist old tests:
```
https://github.com/tc39/test262/blob/master/test/language/function-code/S10.2.1_A5.2_T1.js
https://github.com/tc39/test262/blob/master/test/language/function-code/S10.2.1_A5.1_T2.js
```

[Adam Klein]

See step 27 of https://tc39.github.io/ecma262/#sec-functiondeclarationinstantiation. Basically, if a var-declared name has the same name as a parameter (and there aren't any parameter expressions, i.e., default parameter values or destructuring), the var declaration is ignored.

[Mats Åhlberg]

When running code through uglify-js it generates this type of code which breaks.
 var b=async (t,a,r)=> {
 console.log(a);
   var a = a;
   console.log(t);
   console.log(a);
   console.log(r);
   return { t, a, r };
  };
  b(1, 2, 3);

replacing the var a = 1; with a=a works.
the first console log says undefined when the var is left.

the code works in firefox and chrome. 

so is this a bug in webkit or the other browsers?

[Radar WebKit Bug Importer]

<rdar://problem/113883888>

[Alexey Shvayka]

*** This bug has been marked as a duplicate of bug 223533 ***