content/browser/debugger/manual_tests/debugger-pause-on-for-in-statements.html

<script>
function test()
{
    debugger;
}

var object = { test: 1 };

for (var property in object)
    test();
</script>

<p>To test, open the DevTools (Ctrl+Shift+I) and reload the page. When the debugger breaks, select
the (anonymous function) node in the call stack, you should see the execution line on the call to test().
<br><br>
Also set a breakpoint on the call to test(), and reload. It should break before the test() function is called.