一共三种写法 如下：
for (var i = 0; i < arr.length; i++)
for (var i in arr)
for (var i = 0, len = arr.length; i < len; i++)
先看下运行测试的界面：
如此看来，执行效率最快的是第三种：
for (var i = 0, len = arr.length; i < len; i++)
先声明变量，再进行循环判断，效率远比遍历数组高得多，也比循环取得数组长度的效率快得多！
下面是页面源代码：
<html>
<body>
<script type="text/javascript">
function makeArr(num) {
var arr = [];
for (var i = 0; i<num; i++){
arr.push('abc');
}
arr.join('');
return arr;
}
var num = 10000000;
function function1() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i = 0; i < arr.length; i++) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
function function2() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i in arr) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
function function3() {
var arr = makeArr(num);
var start = (new Date()).valueOf();
var count = 0;
for (var i = 0, len = arr.length; i < len; i++) {
count++;
}
var end = (new Date()).valueOf();
console.log('Count: '+count+' times');
console.log('Time spent: '+(end - start));
}
</script>
for (var i = 0; i < arr.length; i++) <br />
<button onClick="function1();">loop 1</button><br />
<p id="f1"></p>
for (var i in arr)<br />
<button onClick="function2();">loop 2</button><br />
<p id="f2"></p>
for (var i = 0, len = arr.length; i < len; i++)<br />
<button onClick="function3();">loop 3</button><br />
<p id="f3"></p>
</body>
</html>
