一共三種寫法 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++)
先看下運行測試的界面：
再看下對應的三個結果，三種寫法執行的效率是有很大不同的：
如此看來，執行效率最快的是第三種：
view sourceprint?for (var i = 0, len = arr.length; i < len; i++)
先聲明變量，再進行循環判斷，效率遠比遍歷數組高得多，也比循環取得數組長度的效率快得多！
下面是頁面源代碼：
view sourceprint?<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>