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？
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
var beginTime = new Date();
for ( i = 0; i < count; i++ ) {
o = eval( "(" + jsonString + ")" );
}
Console.output( "eval:" + ( new Date() - beginTime ) ); var beginTime = new Date();
for ( i = 0; i < count; i++ ) {
o = new Function( "return " + jsonString )();
}
Console.output( "new Function:" + ( new Date() - beginTime ) ); 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!" );
} 
*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：
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。
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：
//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 ) );<!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 id="consoleRegion"></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>