In the case of closure, yes you will take a small performance hit. But the example given is still invalid because the only scope present is the function-scope (for-loop doesn't create new scope).

For example, if you ran the code below in FireBug you'll get the same results (minor variance aside).

function foo() { var a = 0; for (var i=0, j=a; i<10000000; i++) { var x = j+i; } }

function bar() { var a = 0; for (var i=0; i<10000000; i++) { var x = a+i; } }

(function() { var start = new Date().getTime(), end; foo(); end = new Date().getTime(); console.log(end-start); })();

(function() { var start = new Date().getTime(), end; bar(); end = new Date().getTime(); console.log(end-start); })();