一共三种写法 如下：
for (var i = 0; i < arr.length; i++)
for (var i = 0, len = arr.length; i < len; i++)
先看下运行测试的界面：
再看下对应的三个结果，三种写法执行的效率是有很大不同的：
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>
