该demo来自：
<html>
<head>
<script src="jquery.min.js"></script>
<script>
var sum1 = 0, sum2 = 0, sum3 = 0; len = 5;
var arr = [];
for (var i = 0; i < len; i++) {
arr.push(i)
}
for (var i = 0; i < len; i++) {
setTimeout(function () {
sum1 += arr[i];
}, 0);
}
$.each(arr, function (i, v) {
setTimeout(function () {
sum2 += v;
}, 0);
});
for (var i = 0; i < len; i++) {
sum3++;
}
//在len=2与len=200000时，sum1、sum2、sum3的值各是多少
console.log(sum1);
console.log(sum2);
console.log(sum3);
</script>
</head>
<body></body>
</html>