jquery中ajax处理跨域的三大方式

 header("Access-Control-Allow-Origin:*");
 /*星号表示所有的域都可以接受，*/
 header("Access-Control-Allow-Methods:GET,POST");

 /*当前网址是localhost:3000*/
 js代码
 
 $.ajax({
 type:"get",
 url:"http://localhost:3000/showAll",/*url写异域的请求地址*/
 dataType:"jsonp",/*加上datatype*/
 jsonpCallback:"cb",/*设置一个回调函数，名字随便取，和下面的函数里的名字相同就行*/
 success:function(){
 。。。
 }
 });

 /*而在异域服务器上，*/
 app.js
 app.get('/showAll',students.showAll);/*这和不跨域的写法相同*/
 
 
 /*在异域服务器的showAll函数里，*/

 var db = require(".https://img.qb5200.com/download-x/database");

 exports.showAll = function(req,res){

 /**设置响应头允许ajax跨域访问**/
 res.setHeader("Access-Control-Allow-Origin","*");
 /*星号表示所有的异域请求都可以接受，*/
 res.setHeader("Access-Control-Allow-Methods","GET,POST");

 var con = db.getCon();
 con.query("select * from t_students",function(error,rows){
 if(error){
 console.log("数据库出错："+error);
 }else{
 /*注意这里，返回的就是jsonP的回调函数名+数据了*/
 res.send("cb("+JSON.stringify(r)+")");
 }
 });
 }

//跨域（可跨所有域名）
$.getJSON("http://user.hnce.com.cn/getregion.aspx?id=0&jsoncallback=?",function(json){ 
//要求远程请求页面的数据格式为： ?(json_data) //例如： //?([{"_name":"湖南省","_regionId":134},{"_name":"北京市","_regionId":143}]) alert(json[0]._name); 
}); 

$.getJSON("http://user.hnce.com.cn/getregion.aspx?id=0&jsoncallback=?",function(json){ 
//要求远程请求页面的数据格式为： ?(json_data) //例如： //?([{"_name":"湖南省","_regionId":134},{"_name":"北京市","_regionId":143}]) alert(json[0]._name); 
}); 

<script src="http://common.cnblogs.com/script/jquery.js" type="text/javascript"></script> 
<script type="text/javascript"> 
//跨域（可跨所有域名） 
$.getJSON("http://e.hnce.com.cn/tools/ajax.aspx?jsoncallback=?", { id: 0, action: 'jobcategoryjson' }, function(json) { alert(json[0].pid); alert(json[0].items[0]._name); });　 
 </script> 

<script src="http://common.cnblogs.com/script/jquery.js" type="text/javascript"></script> 
<script type="text/javascript"> 
//跨域（可跨所有域名） 
$.getJSON("http://e.hnce.com.cn/tools/ajax.aspx?jsoncallback=?", { id: 0, action: 'jobcategoryjson' }, function(json) { alert(json[0].pid); alert(json[0].items[0]._name); });　 
 </script> 

<script type="text/javascript" src="http:/https://img.qb5200.com/download-x/domain2.com/getjson?jsonp=parseResponse"> </script> 
响应值：parseResponse({"Name": "Cheeso", "Rank": 7}) 
<script type="text/javascript" src="http:/https://img.qb5200.com/download-x/domain2.com/getjson?jsonp=parseResponse"> </script> 
响应值：parseResponse({"Name": "Cheeso", "Rank": 7}) 

getJSON: function( url, data, callback ) {
 return jQuery.get(url, data, callback, "json");
 },
 

 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
 });

 // 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;
 }