[Rails 3.2.8 & ExecJS 1.4.0]Getting syntax errors, but how can I debug

Question

For example I have the following code:

function f(p) {
 for(var i=0;i<1;i++)
  {
   p=unescape(p);
  }
  return p;
 }

 var c='something';

When I past it in the firebug console it returns undefined. That's fine by now

When I try to run it with ExecJS. ExecJS returns: ExecJS::RuntimeError: SyntaxError: Unexpected token var

I can't get a clue where to start searching for the problem. Does anyone has an idea?