Basic use of ajax

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

　　为了方便使用JQ为我们封装好了一个ajax来方便我们的使用：

$.ajax({
      type:"post",//请求方式
      url:"url",//请求地址
      data:"...",//发送至服务端的数据
      contentType:"...",//设置发送数据的类型如果data是一个json字符串这里要设置为application/json
      success:function(data){...},//请求成功的回调函数data可看做是服务器返回的数据
      error:function(){...}//请求失败的回调函数
    });

　　或者：

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

回调函数中的data即为服务器返回的数据的一个代理，直接使用即可。

为了简化我们的开发JQ提供了一些全局设置函数包括$.ajaxSetup()、$.()ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()。

$.ajaxSetup()用来设置基本的参数例如：

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

我们在使用$.ajax时可以直接这样设置：

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

最终等价于：

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

　　$().ajaxStart()、$().ajaxStop()、$().ajaxComplete()、$().ajaxError()、$().ajaxSuccess()、$().ajaxSend()都是用来设置一些全局回调函数的。例如我们在提交数据时为了防止多次提交我们需要在发送请求时产生一个loading遮罩在数据发送完成后取消遮罩，如果在每一次ajax请求时我们都设置一次就会很麻烦，此时我们就可以用全局回调函数来简化我们的操作：

　　利用全局事件在请求开始时产生一个遮罩在请求完成时取消遮罩：

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