Js极客之路 - 优化操作（性能优化）

1．因为每次For循环都会计算一次arr.length，所以有必要存储数组长度以减少计算。针对这篇文章（http://www.crimx.com/2015/04/21/should-array-length-be-cached-or-not/），V8引擎好像已经帮我们做了不变数据的缓存，不过个人认为还是很有必要的，像他说的，假若只有V8引擎做了此事（V8开发者有这样说吗？），我们做缓存，对于V8就当帮它咯，对于其他就是优化咯。

    //bad    var arr=[1,2,3,...];        for(var i=0; i<arr.length; i++){        ...code    }        //good    var arr=[1,2,3,...],        len=arr.length,        i=0;            for(; i<len; i++){        ...code    }

2．将字符串转变成数字的几种方法。

    //第一种：    var a='12';    console.log(typeof a);        //string    a=parseInt(a);                //parseInt()、Math.floor()等    console.log(typeof a);        //number        //第二种：    var a='12';    a=a-0;        //-、*、/等    console.log(typeof a);        //number        //第三种（参考地址-http://www.Vevb.com.cn/js/PRo_js_Operators_unary.asp）：    var a='12';    a = +a ;        //+、-、~等    console.log(typeof a);        //number        //第四种：    var a='12';    a = a >> 0 ;        //>>、>>>等    console.log(typeof a);        //number        听说第二种和第三种是性能快点，或许可以参照《将数字转变成字符串的几种方法》下别人的话。    第四种：按位操作是在数值表示的最底层执行操作，因此速度更快。计算过程参考：        ①：https://msdn.microsoft.com/zh-cn/library/5s9e947e(v=vs.94).aspx        ②：http://zhidao.baidu.com/link?url=lHM_2XsAX2N3zmaDwhA8IUivdkjhBO-9RLYg-qLRfTuh7qo2wJYaEZXi5mlu4VWPMSNtBL404448uJBxrrfK_Y-E4WDdhSgg31ofHAj72nC    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