Ajax is a technology that must be used in our development. Ajax is asynchronous javascript and xml. But now we usually use json to complete data interaction. Ajax has a single responsibility, which is the interaction of data. Sending data and receiving data is its core. Function is also the only function.

　The implementation of ajax relies on XMLHttpRequest. Its basic usage is as follows:

var xhr;
window.XMLHttpRequest?xhr = new XMLHttpRequest():xhr = new ActiveXObject("Microsoft.XMLHTTP");
xhr.open("get","demo!register.action?name=zt&age=23",true);
xhr.send(null);
xhr.onreadystatechange = function(){
　　if(xhr.readyState==4&&xhr.status==200){
   　　alert(JSON.parse(xhr.responseText));
　　}
}

　The responsibility of ajax is to send and receive data. Our basic usage process is:

　1. Obtain an XMLHttpRequest object

　2. Send data

　3. Receive and process the data returned by the server

According to the above steps to implement an asynchronous request for data, first obtain an xhr object. In modern browsers, we can obtain an xhr object directly through instantiation: var xhr = new XMLHttpRequest( ); In IE5 and IE6 we must use ActiveXObject to obtain the xhr object: var xhr = new ActiveXObject("Microsoft.XMLHTTP").

At this point we have obtained the xhr object. The next step is to send data. The method of sending data is executed through the xhr.open() method. xhr.open() can receive 5 parameters. We often use the first three:

It is asynchronous, so we generally use the asynchronous method. The third parameter is set to true (true means to make an asynchronous request, false means to make a synchronous request)

　The xhr.open() method only prepares a request, and will not do it after calling open To communicate with the server, communication with the server will not start until the send() function is called. The parameters of the send() function will be sent to the server as the request body. If we specify the request mode in the open() function as get, usually we set send() to xhr.send(null). If we want to send data through the request body, we must set the request mode of the open() function to post. At the same time, use the data we need to send as the parameter of the send() function: xhr.send(param). After calling the send() function, communication with the server begins.

　 All settings for xhr should be set before the send() function:

xhr.open(...);
  xhr.setRequestHeader(...);
  xhr.overrideMimeType(...);
  xhr.onreadystatechange = function(){...};
  xhr.send(...);

　 But since xhr.onreadystatechange is an event, it can also be executed after send(). For the sake of readability, we Generally, the settings for xhr are placed before the send() function.

　 After send(), you can monitor the status of this request through xhr.readyState and xhr.status. If xhr.readyState==4&&xhr.status==200 is met, the request is successful:

　When the request is successful, we can obtain the data returned by the server through xhr.responseText. It should be noted that xhr.responseText is a string.

ajax common API

　The above request process is the most basic request process. The xhr object also has several frequently used methods: xhr.abort(), xhr.setRequestHeader(), xhr.overrideMimeType().

　 xhr.abort(): Terminate a request, just call it directly without setting parameters

xhr.abort()

　 xhr.setRequestHeader(): Set the request header sent:

xhr.setRequestHeader("Content- Type","application/json; charset=utf-8")

　The first parameter represents the header to be set, and the second parameter represents the value of the header to be set. xhr.setRequestHeader() must be between xhr.open() and xhr.send(), otherwise an exception will be thrown. At the same time, the first parameter of xhr.setRequestHeader() is not case-sensitive as long as we write it in letters. The setting can be successful, but for readability we need to set it to the correct format.

　 xhr.overrideMimeType(): Rewrite the Content-Type of the response header:

xhr.overrideMimeType('text/plain; charset=utf-8')

xhr.overrideMimeType() must also be set in xhr.send ()Before.

Usage of JSON.parse() and JSON.stringify()

JSON.parse() is used to convert an object into a string, and JSON.stringify() is used to convert a string into an object. In the process of using ajax for data interaction, most of the data returned is a string in JSON format. If the server returns data to us, we need to use JSON.parse() to parse the returned data (xhr.responseText That is the data returned by the server):

xhr.onreadystatechange = function(){
　　if(xhr.readyState==4&&xhr.status==200){        
　　var data = JSON.parse(xhr.responseText);
　　}
}

In the process of sending data using the post method, if it is not a file upload, usually a JSON data is transmitted. If you want to successfully send it to the background, you need to use JSON.stringify( ) to convert the JSON object into a string, and the Content-Type must be set to application/json:

var sendData = {name:"zt",age:23};
...
xhr.setRequestHeader("Content-Type","application/json; charset=utf-8");
xhr.send(JSON.stringify(sendData));

In addition, JSON.parse() and JSON.stringify() can be used to implement the deep copy function of an object:

var sendData = {name:"zt",age:23};
var copyData = JSON.parse(JSON.stringify(sendData));

$.ajax基本使用

　　為了方便使用JQ為我們封裝好了一個ajax來方便我們的使用：

$.ajax({
      type:"post",//請求方式
      url:"url",//請求地址
      data:"...",//發(fā)送至服務(wù)端的數(shù)據(jù)
      contentType:"...",//設(shè)置發(fā)送數(shù)據(jù)的類型如果data是一個json字符串這里要設(shè)置為application/json
      success:function(data){...},//請求成功的回調(diào)函數(shù)data可看做是服務(wù)器返回的數(shù)據(jù)
      error:function(){...}//請求失敗的回調(diào)函數(shù)
    });

　　或者：

$.ajax({
      type:"post",
      url:"url",
      data:"...",
      contentType:"...",
  })
  .done(function(data){...})
  .fail(function(){...});

回調(diào)函數(shù)中的data即為服務(wù)器返回的數(shù)據(jù)的一個代理，直接使用即可。

為了簡化我們的開發(fā)JQ提供了一些全局設(shè)置函數(shù)包括$.ajaxSetup()、$.()ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()。

$.ajaxSetup()用來設(shè)置基本的參數(shù)例如：

  $.ajaxSetup({
      type:"post",
      contentType:"application/json; charset=utf-8"
  });

我們在使用$.ajax時可以直接這樣設(shè)置：

$.ajax({
      url:"",
      success:function(){...},
      error:function(){...}
  })

最終等價于：

$.ajax({
      type:"post",
      contentType:"application/json; charset=utf-8",
      url:"",
      success:function(){...},
      error:function(){...}
  })

　　$().ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()都是用來設(shè)置一些全局回調(diào)函數(shù)的。例如我們在提交數(shù)據(jù)時為了防止多次提交我們需要在發(fā)送請求時產(chǎn)生一個loading遮罩在數(shù)據(jù)發(fā)送完成后取消遮罩，如果在每一次ajax請求時我們都設(shè)置一次就會很麻煩，此時我們就可以用全局回調(diào)函數(shù)來簡化我們的操作：

　　利用全局事件在請求開始時產(chǎn)生一個遮罩在請求完成時取消遮罩：

$(document).ajaxStart(function(){
      loadingMask.show();
  });
  $(document).ajaxComplete(function(){
      loadingMask.hide();
  });