Javascript中for循环的三种写法的效率对比_建站学-个人建站指南,网页制作,网站设计,网站制作教程

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：

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>

(责任编辑：admin)

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Javascript中for循环的三种写法的效率对比