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

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())

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.

Fixed by bug 1263355.