一共三种写法 如下：
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for (var
i = 0; i < arr.length; i++) |
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for (var
i in
arr) |
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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：
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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：
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<html> |
<body> |
<script type="text/网页特效">
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|
function
makeArr(num) { |
var
arr = []; |
for
(var
i = 0; i<num; i++){ |
arr.push('abc');
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} |
arr.join('');
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return
arr; |
} |
var num = 10000000;
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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');
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console.log('Time spent: '+(end - start));
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} |
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');
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console.log('Time spent: '+(end - start));
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} |
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');
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console.log('Time spent: '+(end - start));
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} |
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</script> |
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for (var
i = 0; i < arr.length; i++) <br /> |
<button onClick="function1();">loop 1</button><br />
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<p id="f1"></p>
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for (var
i in
arr)<br /> |
<button onClick="function2();">loop 2</button><br />
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<p id="f2"></p>
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for (var
i = 0, len = arr.length; i < len; i++)<br /> |
<button onClick="function3();">loop 3</button><br />
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<p id="f3"></p>
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</body> |
</html>
来源：CSDN
作者：louislam0620
链接：https://blog.csdn.net/louislam0620/article/details/49363857