Detailed explanation of $.ajax() method parameters

1.url:

is required to be a String type parameter, (default is the current page address) to which the request is sent.

2.type:
Requires parameters of String type, and the request method (post or get) defaults to get. Note that other http request methods such as put and delete can also be used, but are only supported by some browsers.

3.timeout:
Requires a parameter of type Number and sets the request timeout (milliseconds). This setting will override the global setting of the $.ajaxSetup() method.

4.async:
Requires a Boolean type parameter, the default setting is true, and all requests are asynchronous requests. If you need to send synchronous requests, set this option to false. Note that a synchronous request will lock the browser, and the user must wait for the request to complete before other operations can be performed.

5.cache:
Requires parameters of Boolean type, defaults to true (when dataType is script, defaults to false), set to false Request information will not be loaded from the browser cache.

6.data:
Requires parameters of type Object or String, data sent to the server. If it is not a string, it will be automatically converted to string format. The get request will be appended to the url. To prevent this automatic conversion, check the processData option. The object must be in key/value format, for example {foo1:"bar1",foo2:"bar2"} is converted to &foo1=bar1&foo2=bar2. If it is an array, JQuery will automatically assign the same name to different values. For example, {foo:["bar1","bar2"]} is converted to &foo=bar1&foo=bar2.

7.dataType:
Requires parameters of String type, expected data type returned by the server. If not specified, JQuery will automatically return responseXML or responseText based on the http package mime information and pass it as a callback function parameter. The available types are as follows:
xml: Returns an XML document that can be processed with JQuery.
html: Returns plain text HTML information; the included script tag will be executed when inserted into the DOM.
script: Returns plain text JavaScript code. Results are not automatically cached. Unless cache parameters are set. Note that when making remote requests (not under the same domain), all post requests will be converted into get requests.
json: Return JSON data.
jsonp: JSONP format. When calling a function using the SONP form, such as myurl?callback=?, JQuery will automatically replace the last "?" with the correct function name to execute the callback function.
text: Returns a plain text string.

8.beforeSend:
is required to be a parameter of Function type. You can modify the function of the XMLHttpRequest object before sending the request, such as adding a custom HTTP header. If false is returned in beforeSend, this ajax request can be canceled. The XMLHttpRequest object is the only parameter.

            function(XMLHttpRequest){
               this;   //调用本次ajax请求时传递的options参数
            }

9.complete：
is required to be a parameter of Function type, the callback function called after the request is completed (the request is successful or Called on failure). Parameters: XMLHttpRequest object and a string describing the successful request type.

          function(XMLHttpRequest, textStatus){
             this;    //调用本次ajax请求时传递的options参数
          }

10.success: The requirement is a parameter of Function type. The callback function called after the request is successful has two parameters.
           (1) Data returned by the server and processed according to the dataType parameter.
(2) Describe the string of the state.

         function(data, textStatus){
            //data可能是xmlDoc、jsonObj、html、text等等
            this;  //调用本次ajax请求时传递的options参数
         }

11.error:
is required to be a parameter of Function type, the function to be called when the request fails. This function has three parameters, namely XMLHttpRequest object, error message, and captured error object (optional). The ajax event function is as follows:

       function(XMLHttpRequest, textStatus, errorThrown){
          //通常情况下textStatus和errorThrown只有其中一个包含信息
          this;   //调用本次ajax请求时传递的options参数
       }

12.contentType:
is required to be a String type parameter. When sending information to the server, the content encoding The type defaults to "application/x-www-form-urlencoded". This default value is suitable for most applications.

13.dataFilter：
Requires parameters of Function type, a function that preprocesses the original data returned by Ajax. Provide two parameters: data and type. data is the original data returned by Ajax, and type is the dataType parameter provided when calling jQuery.ajax. The value returned by the function will be further processed by jQuery.

            function(data, type){
                //返回处理后的数据
                return data;
            }

14.dataFilter：
Requires parameters of Function type, a function that preprocesses the original data returned by Ajax. Provide two parameters: data and type. data is the original data returned by Ajax, and type is the dataType parameter provided when calling jQuery.ajax. The value returned by the function will be further processed by jQuery.

            function(data, type){
                //返回处理后的数据
                return data;
            }

15.global:
is required to be a Boolean type parameter, and the default is true. Indicates whether to trigger the global ajax event. Setting to false will not trigger global ajax events, ajaxStart or ajaxStop can be used to control various ajax events.

16.ifModified：
要求为Boolean类型的参数，默认为false。仅在服务器数据改变时获取新数据。服务器数据改变判断的依据是Last-Modified头信息。默认值是false，即忽略头信息。

17.jsonp：
要求为String类型的参数，在一个jsonp请求中重写回调函数的名字。该值用来替代在"callback=?"这种GET或POST请求中URL参数里的"callback"部分，例如{jsonp:'onJsonPLoad'}会导致将"onJsonPLoad=?"传给服务器。

18.username：
要求为String类型的参数，用于响应HTTP访问认证请求的用户名。

19.password：
要求为String类型的参数，用于响应HTTP访问认证请求的密码。

20.processData：
要求为Boolean类型的参数，默认为true。默认情况下，发送的数据将被转换为对象（从技术角度来讲并非字符串）以配合默认内容类型"application/x-www-form-urlencoded"。如果要发送DOM树信息或者其他不希望转换的信息，请设置为false。

21.scriptCharset：
要求为String类型的参数，只有当请求时dataType为"jsonp"或者"script"，并且type是GET时才会用于强制修改字符集(charset)。通常在本地和远程的内容编码不同时使用。

案例代码：

$(function(){
    $('#send').click(function(){
         $.ajax({
             type: "GET",
             url: "test.json",
             data: {username:$("#username").val(), content:$("#content").val()},
             dataType: "json",
             success: function(data){
                         $('#resText').empty();   //清空resText里面的所有内容
                         var html = ''; 
                         $.each(data, function(commentIndex, comment){
                               html += '<p></p><h6 id="comment-username">' + comment['username']                                         + ':</h6><p></p>';
                         });
                         $('#resText').html(html);
                      }
         });
    });
});

22.顺便说一下$.each()函数:
$.each()函数不同于JQuery对象的each()方法，它是一个全局函数，不操作JQuery对象，而是以一个数组或者对象作为第1个参数，以一个回调函数作为第2个参数。回调函数拥有两个参数：第1个为对象的成员或数组的索引，第2个为对应变量或内容。

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