Summary of relevant knowledge points of ajax in jQuery

 // IE9 and above // ​​const xhr = new XMLHttpRequest()
 // IE9 and below // const xhr = new ActiveXObject('Mricosoft.XMLHTTP')

 // For this compatible writing method, we can use a function to encapsulate function createXHR() {
 var req = null;
 if (window.XMLHttpRequest) {
 // If you have this XMLHttpRequest object, just use req = new XMLHttpRequest();
 } else {
 // Otherwise, use the IE8 or later method req = new ActiveXObject("Microsoft.XMLHTTP");
 }
 return req;
 }
 // Step 1: Create an ajax object var xhr = createXHR(); // This way you get an ajax object // Step 2: Configure network request information xhr.open('get', './demo.php', true)
 // xhr.open('get/post', 'To which address do you want to send the network request', synchronous or asynchronous, the default is true for asynchronous and false for synchronous)
 // If the get request has parameters, they need to be concatenated after the address, for example './demo.php?id=2&name=sanqi'
 // If it is a post request, the parameters should be placed in send(), for example: xhr.send('id=2&name=sanqi')
 // Step 3: Send network request xhr.send() //
 // Part 4: Determine the response status and get the data xhr.onreadyStateChange = function () {
 // This event is triggered every time the readyState changes // We will check here whether the value of readyState is 4
 // And the http status code is 200 ~ 299
 if (xhr.readyState === 4 && /^2\d{2|$/.test(xhr.status)) {
 // This means the verification is passed // We can get the content of the response from the server console.log(xhr.response);
 }
 }

<?php
header('content-type:text/html;charset=utf-8;');
 $id = $_REQUEST['id'];
 $name = $_REQUEST['name'];
 $arr = [
 'id' => $id,
 'name' => $name
 ];
 echo json_encode($arr);
?>

  $.get('./test.php','id=999&name=三七安',function (res) { 
  //The first parameter is the requested address //The second parameter is the data to be sent to the server //The third parameter is the callback function when successful, which contains the data returned by the service //The fourth parameter is the data format we want to get: there are several options: json, text, html, xml, script
  console.log(res);
  },'json')

	 $.post('./test.php',{id:123,name:'post request'},function (res) { 
	 //The other parameters here are the same as those of the get request //Note that I write the transmission data here in object format, and the request can be sent successfully, that is to say //Whether it is a get request or a post request, if you want to send data to the server, you can use either string format or object format console.log(res);
  },'json')

 // Use the $.ajax method // It only accepts one parameter, which is an object. This object performs all configurations for the current request. $.ajax({
  url: './ajax', // Required, requested address type: 'GET', // Optional, request method, default is GET (ignore case)
  data: {}, // Optional, the parameter to be carried when sending the request dataType: 'json', // Optional, the data type of the expected return value, the default is string
  async: true, // Optional, whether to enable asynchronous operation, default is true
  success() {}, // Optional, callback function for success error() {}, // Optional, callback function for failure cache: true, // Optional, whether to cache, default is true
  context: div, // Optional, this in the callback function points to, the default is the ajax object status: {}, // Optional, execute the function according to the corresponding status code timeout: 1000, // Optional, timeout event })

$.ajax({
  type: "get",
  url: "./test.php",
  data: {
   id:000,
   name:'Demo of sending $.ajax request'
  },
  dataType: "json",
  success: function (response) {
   console.log(response);
  }
  });

$(window).ajaxStart(function () {
 console.log('A request has started')
})

$(window).ajaxSend(function () {
 console.log('One is about to be sent')
})

$(window).ajaxSuccess(function () {
 console.log('A request was successful')
})

$(window).ajaxError(function () {
 console.log('A request failed')
})

$(window).ajaxComplete(function () {
 console.log('A request has been completed')
})

$(window).ajaxStop(function () {
 console.log('A request has ended')
})