<script>
var s;

var date = new Date;
for (var i = 0; i < 100000; i++) s = "ss";
alert(new Date - date);  //79

date = new Date;
for (var i = 0; i < 100000; i++) window.s = "tt";
alert(new Date - date);   //735

alert(s);   //tt(注意:不是ss)
</script>