<script type="text/javascript" src="http://domain2.com/getjson?jsonp=parseResponse"> </script> 响应值：parseResponse({"Name": "Cheeso", "Rank": 7}) <script type="text/javascript" src="http://domain2.com/getjson?jsonp=parseResponse"> </script> 响应值：parseResponse({"Name": "Cheeso", "Rank": 7})
这种方式被称作JsonP;(如果链接已经失效请点击这里:JSONP);即：JSON with padding 上面提到的前缀就是所谓的“padding”。那么jQuery里面是怎么实现的呢？
貌似并没有<Script>标记的出现！？OKay，翻看源码来看：
页面调用的是getJSON：
getJSON: function( url, data, callback ) { return jQuery.get(url, data, callback, "json"); },
继续跟进
get: function( url, data, callback, type ) { // shift arguments if data argument was omited if ( jQuery.isFunction( data ) ) { type = type || callback; callback = data; data = null; } return jQuery.ajax({ type: "GET", url: url, data: data, success: callback, dataType: type });
跟进jQuery.ajax，下面是ajax方法的代码片段：
// Build temporary JSONP function if ( s.dataType === "json" && (s.data && jsre.test(s.data) || jsre.test(s.url)) ) { jsonp = s.jsonpCallback || ("jsonp" + jsc++); // Replace the =? sequence both in the query string and the data if ( s.data ) { s.data = (s.data + "").replace(jsre, "=" + jsonp + "$1"); } s.url = s.url.replace(jsre, "=" + jsonp + "$1"); // We need to make sure // that a JSONP style response is executed properly s.dataType = "script"; // Handle JSONP-style loading window[ jsonp ] = window[ jsonp ] || function( tmp ) { data = tmp; success(); complete(); // Garbage collect window[ jsonp ] = undefined; try { delete window[ jsonp ]; } catch(e) {} if ( head ) { head.removeChild( script ); } }; } if ( s.dataType === "script" && s.cache === null ) { s.cache = false; } if ( s.cache === false && type === "GET" ) { var ts = now(); // try replacing _= if it is there var ret = s.url.replace(rts, "$1_=" + ts + "$2"); // if nothing was replaced, add timestamp to the end s.url = ret + ((ret === s.url) ? (rquery.test(s.url) ? "&" : "?") + "_=" + ts : ""); } // If data is available, append data to url for get requests if ( s.data && type === "GET" ) { s.url += (rquery.test(s.url) ? "&" : "?") + s.data; } // Watch for a new set of requests if ( s.global && ! jQuery.active++ ) { jQuery.event.trigger( "ajaxStart" ); } // Matches an absolute URL, and saves the domain var parts = rurl.exec( s.url ), remote = parts && (parts[1] && parts[1] !== location.protocol || parts[2] !== location.host); // If we're requesting a remote document // and trying to load JSON or Script with a GET if ( s.dataType === "script" && type === "GET" && remote ) { var head = document.getElementsByTagName("head")[0] || document.documentElement; var script = document.createElement("script"); script.src = s.url; if ( s.scriptCharset ) { script.charset = s.scriptCharset; } // Handle Script loading if ( !jsonp ) { var done = false; // Attach handlers for all browsers script.onload = script.onreadystatechange = function() { if ( !done && (!this.readyState || this.readyState === "loaded" || this.readyState === "complete") ) { done = true; success(); complete(); // Handle memory leak in IE script.onload = script.onreadystatechange = null; if ( head && script.parentNode ) { head.removeChild( script ); } } }; } // Use insertBefore instead of appendChild to circumvent an IE6 bug. // This arises when a base node is used (#2709 and #4378). head.insertBefore( script, head.firstChild ); // We handle everything using the script element injection return undefined; }
上面的代码第1行到第10行：判断是JSON类型调用，为本次调用创建临时的JsonP方法，并且添加了一个随机数字，这个数字源于用日期值；
关注第14行，这一行相当关键，注定了我们的结果最终是<Script> ;然后是构造Script片段，第95行在Head中添加该片段，修成正果；
不仅仅是jQuery,很多js框架都是用了同样的跨域方案，这就是getJSON跨域的原理。