前端性能优化：循环优化二，循环展开

Posted 2022-12-11 xuexiaodong2009

这种方式就是把循环展开，据说可以提高性能，但测试效果不明显，在谷歌50中展开反倒变慢了，IE10这种变快了。看来实际使用中中还是需要测试

测试代码：

 function doProcess(value1, value2) 
        return value1 + value2;
    
    function createArr() 
        var t = [];
        for (var i = 0; i < 3000; i++) 
            t[i] = i;
        
        return t;
    
    /*原始循环*/
    function Test1() 
        var t = createArr();
        var date1 = new Date().getTime();
        var total = 0;
        var length = t.length;
        for (var i = 0; i < length; i++) 
            total = doProcess(total, t[i]);
        
        var date12 = new Date().getTime();
        console.log("1Test" + ((date12 - date1)));     
    
    /*循环展开*/
    function Test2() 
        var t = createArr();
        var date1 = new Date().getTime();
        var length = t.length;
        var iteration = Math.floor(length / 8);
        var leftover = length % 8;
        var total = 0;
        var i = 0;
        if (leftover > 0) 
            do 
                total = doProcess(total, t[i++]);
            
            while (--leftover > 0)
        
        do 
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
            total = doProcess(total, t[i++]);
        
        while (--iteration > 0)
        var date12 = new Date().getTime();
        console.log("2Test" + ((date12 - date1)));      
    

其实展开也是一种思路，对于具体的问题还需要具体的分析，根据业务在进行改写，例如：

 /*原始循环*/
    function Test1() 
        var t = createArr();
        var date1 = new Date().getTime();
        var total = 0;
		var total2 = 0;
        var length = t.length;
        for (var i = 0; i < length; i++) 
		    if(i%2)
			
			  total2 = doProcess(total2, t[i]);
			
			else
			total = doProcess(total, t[i]);
			            
        
        var date12 = new Date().getTime();
        console.log("1Test" + ((date12 - date1)));  
        console.log(total2+"Test" +total); 		
    
    /*循环展开*/
   function Test2() 
        var t = createArr();
        var date1 = new Date().getTime();
        var total = 0;
		var total2 = 0;
        var length = t.length;
        for (var i = 0; i < length; ) 
		    total2 = doProcess(total2, t[i]);
             i+=2;			
        
		 for (var i = 0; i < length; ) 
		     total = doProcess(total, t[i]);
             i+=2;			
        
        var date12 = new Date().getTime();
        console.log("2Test" + ((date12 - date1)));    
        console.log(total2+"Test" +total); 			
    

因为分开后，少了一次判断，速度就快了，如果有很多不同的处理，就会快很多。