1022967 - Separate environment for default parameter and function body if default parameter contains expression

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Tooru Fujisawa [:arai]]

User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:30.0) Gecko/20100101 Firefox/30.0 (Beta/Release)
Build ID: 20140605174243

Steps to reproduce:

(Separated from 1018628)

Evaluate following code in Firefox 29:

> var z = 10;
> function f(b=(() => z) {
>   var z = 20;
>   return b();
> }
> f()



Actual results:

|f()| returns |20|.



Expected results:

|f()| should return |10|.

This is caused by that variable |z| inside function body is visible from closure in default parameter, and the closure captures it instead of |z| in outer binding.

Related to step 28 of 9.2.13 in the spec:
http://people.mozilla.org/~jorendorff/es6-draft.html#sec-functiondeclarationinstantiation

Comment 1

[David Bruant]

There is a missing parens in your code above (and the >). Repasting one which is easier to copy:

var z = 10;
function f(b=(() => z)) {
  var z = 20;
  return b();
}
console.log(f())

Comment 2

[Tooru Fujisawa [:arai]]

This bug will also have effect for the case when default parameter refers the function declared in the function body.
So I changed the Summary to follow the spec:
  6. Let needsParameterEnvironment be ContainsExpression of formals.

Comment 6

[Shu-yu Guo [:shu]]

Fixed by bug 1263355.