JavaScript 解析Json字符串的性能比较分析代码
我们在使用AJAX来做服务器端和客户端交互的时候，一般的做法是让服务器端返回一段JSON字符串，然后在客户端把它解析成JavaScript对象。
解析时用到的方法一般是eval或者new function，而目前IE8和Firefox3.1又内置了原生的JSON对象（据说会有一定的性能提升）。那我们在实际使用的时候怎样从这三种方法（因为性能问题，不考虑用javascript实现的解析）里面来选择呢？面对众多的浏览器，哪种方式的性能是最好的呢？
一、测试方法
1、首先指定测试次数及JSON字符串代码如下：
var count = 10000, o = null, i = 0, jsonString = '{"value":{"items": [{"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}';2、循环解析并记录时间
eval
代码如下：
var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = eval( "(" + jsonString + ")" ); } Console.output( "eval:" + ( new Date() - beginTime ) );new Function
代码如下：
var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = new Function( "return " + jsonString )(); } Console.output( "new Function:" + ( new Date() - beginTime ) );native
代码如下：
if ( typeof JSON !== "undefined" ) { var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse( jsonString ); } Console.output( "native:" + ( new Date() - beginTime ) ); } else { Console.output( "native:not support!" ); }二、测试对象
选择目前主流的浏览器（不考虑Maxthon一类的外壳），包括IE6、7、8，Firefox2、3、3.1，Chrome，Opera及Safari3、4。三、测试环境
T9300 CPU + 4G RAM + Windows2003，其中IE8使用的是Vista的环境，IE7在另外一台工作机（2G CPU + 2G RAM + Windows2003），考虑到主要是测试浏览器客户端的性能，结果的误差应该能够接受。四、测试结果
*数值越小越好
*在当前列中绿色背景的表示性能最好，红色性能最差：
1、Firefox2、3全部垫底，IE6的性能优于IE7（可能和机器不一致有关），Chrome和Safari4的性能远远超出其它浏览器。
2、不同的浏览器下eval和new Function的性能不一致，总的来说eval更好，但Firefox下new Function的性能是eval的一倍，为了更好的兼容各个浏览器，我们把对JSON的解析单独封装成一个对象来处理：
wrapper
代码如下：
var __json = null; if ( typeof JSON !== "undefined" ) { __json = JSON; } var browser = Browser; var JSON = { parse: function( text ) { if ( __json !== null ) { return __json.parse( text ); } if ( browser.gecko ) { return new Function( "return " + text )(); } return eval( "(" + text + ")" ) } }; var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse( jsonString ); } Console.output( "wrapper:" + ( new Date() - beginTime ) );
由于涉及到调用对象的开销，封装后JSON对象会比单独调用更慢，但它能保证在各个浏览器下使用最适合的方法。
五、结论
解析Json字符串时，不同的浏览器选择不同的方法：- IE6、7使用eval
- IE8使用原生的JSON对象
- Firefox2、3使用new Function
- Safari4使用eval
- 其它浏览器下eval和new Function的性能基本一致
2009.03.23：屏蔽所有Firefox的Add-Ons再进行测试
由于Known在Firefox下运行代码得到了完全不一致的结果，怀疑是Firefox的插件导致，于是禁掉所有插件后（后来表明几乎由Firebug导致），重新在Firefox2、3下测试了一下，结果如下：
这表明Firefox本身的性能并不是象我们先前测试的那样低，在去掉插件后性能还是很不错。但是没有Firebug一类的插件支持，Firefox对我们的吸引力也大大降低了。
2009.03.31：循环中每次使用新的json字符串
根据Oliver的描述，他猜测是由于Safari4和Chrome缓存了eval的结果从而导致它们的测试成绩“虚”高，测试结果证明了他的推测:
从这个结果中我们可以看到，Opera的性能最好，Ie8其次。
主要修改的代码：
代码如下：
//eval 2: var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = eval("(" + '{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}' + ")"); } Console.output( "eval:" + ( new Date() - beginTime ) ); //new Function beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = new Function("return " + '{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}')(); } Console.output( "new Function:" + ( new Date() - beginTime ) ); //native if ( typeof JSON !== "undefined" ) { beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse('{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'); } Console.output( "native:" + ( new Date() - beginTime ) ); } else { Console.output( "native:not support!" ); } //wrapper var __json = null; if ( typeof JSON !== "undefined" ) { __json = JSON; } var browser = Browser; var JSON = { parse: function( text ) { if ( __json !== null ) { return __json.parse( text ); } if ( browser.gecko ) { return new Function( "return " + text )(); } return eval( "(" + text + ")" ) } }; beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse('{"value":{"items": [{"x":' + i + ',"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'); } Console.output( "wrapper:" + ( new Date() - beginTime ) );附：全部代码
代码如下：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>Parse JsonString</title> </head> <body> <div></div> <script type="text/javascript"> //yui var Browser = function() { var o = { ie: 0, opera: 0, gecko: 0, webkit: 0 }; var ua = navigator.userAgent, m; if ( ( /KHTML/ ).test( ua ) ) { o.webkit = 1; } // Modern WebKit browsers are at least X-Grade m = ua.match(/AppleWebKit\/([^\s]*)/); if (m&&m[1]) { o.webkit=parseFloat(m[1]); } if (!o.webkit) { // not webkit // @todo check Opera/8.01 (J2ME/MIDP; Opera Mini/2.0.4509/1316; fi; U; ssr) m=ua.match(/Opera[\s\/]([^\s]*)/); if (m&&m[1]) { o.opera=parseFloat(m[1]); } else { // not opera or webkit m=ua.match(/MSIE\s([^;]*)/); if (m&&m[1]) { o.ie=parseFloat(m[1]); } else { // not opera, webkit, or ie m=ua.match(/Gecko\/([^\s]*)/); if (m) { o.gecko=1; // Gecko detected, look for revision m=ua.match(/rv:([^\s\)]*)/); if (m&&m[1]) { o.gecko=parseFloat(m[1]); } } } } } return o; }(); var Console = { consoleRegion: null, getRegion: function() { if ( this.consoleRegion === null ) { this.consoleRegion = document.getElementById( "consoleRegion" ); } return this.consoleRegion; }, output: function( text ) { this.getRegion().innerHTML += "<br/>" + text; } }; //test code var count = 10000, o = null, i = 0, jsonString = '{"value":{"items": [{"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}, {"x":1,"y":2,"z":3}]},"error":null}'; //eval var beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = eval( "(" + jsonString + ")" ); } Console.output( "eval:" + ( new Date() - beginTime ) ); //new Function beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = new Function( "return " + jsonString )(); } Console.output( "new Function:" + ( new Date() - beginTime ) ); //native if ( typeof JSON !== "undefined" ) { beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse( jsonString ); } Console.output( "native:" + ( new Date() - beginTime ) ); } else { Console.output( "native:not support!" ); } //wrapper var __json = null; if ( typeof JSON !== "undefined" ) { __json = JSON; } var browser = Browser; var JSON = { parse: function( text ) { if ( __json !== null ) { return __json.parse( text ); } if ( browser.gecko ) { return new Function( "return " + text )(); } return eval( "(" + text + ")" ) } }; beginTime = new Date(); for ( i = 0; i < count; i++ ) { o = JSON.parse( jsonString ); } Console.output( "wrapper:" + ( new Date() - beginTime ) ); //alert( o.value.items[0].z ); </script> </body> </html>