Javascript中for循环的三种写法的效率对比

一共三种写法如下：

view sourceprint?for (var i = 0; i < arr.length; i++)

view sourceprint?for (var i in arr)

view sourceprint?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：

view sourceprint?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：

view sourceprint?<html>

<body>

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 />

for (var i in arr)<br />

for (var i = 0, len = arr.length; i < len; i++)<br />

</body>

</html>

补充：web前端 , JavaScript ,