Summary of Ajax for Beginners

Many people have just come into contact with ajax. This article mainly summarizes and organizes some common basic knowledge of Ajax, which is very suitable for beginners. Friends in need can refer to it, and let’s learn together.

1. Introduction to Ajax, advantages and disadvantages, application scenarios and technologies

Introduction to Ajax:

Asynchronous Javascript And XML (Asynchronous JavaScript and XML)

It is not a single technology, but a combination of a series of technologies related to interactive web applications

AJAX is a method used to create rapid Dynamic web technology. AJAX enables web pages to update asynchronously by exchanging small amounts of data with the server in the background. This means that parts of a web page can be updated without reloading the entire page.

Advantages:

1. No page refresh, good user experience.

2. Asynchronous communication, faster response capability.

3. Reduce redundant requests and reduce server burden

4. Based on standardized and widely supported technology, no need to download plug-ins or applets
   

Disadvantages:

1. ajax kills the back button, which destroys the browser's back mechanism.

2. There are certain security issues.

3. The support for search engines is relatively weak.

4. Destroys the exception mechanism of the program.

5. Unable to access directly with URL
   

ajax application scenario

• Scenario 1. Data verification

• Scenario 2. Fetch data on demand

• Scenario 3. Automatically update the page
  

AJAX contains the following five parts:

ajax is not a new technology, but a combination of several original technologies. It is composed of the following technologies.

1. Represented using CSS and XHTML.

2. Use the DOM model for interaction and dynamic display.

3. Data exchange and operation technology, using XML and XSLT

4. Use XMLHttpRequest to communicate asynchronously with the server.

5. Use javascript to bind and call.
   

Among the above technologies, except for the XmlHttpRequest object, all other technologies are based on web standards and have been widely used. Although XMLHttpRequest has not yet been Adopted by W3C, it is already a de facto standard because almost all major browsers currently support it

The first picture especially illustrates traditional Web applications The difference between the structure and the structure of Web applications using AJAX technology

The main difference is actually not JavaScript, not HTML/XHTML and CSS, but the use of XMLHttpRequest to asynchronously send messages to the server Request XML data

Look at the second picture again. In the traditional Web application model, the user experience is fragmented. Click ->Wait->See Go to the new page->click again->and wait. After adopting AJAX technology, most of the calculation work is completed by the server without the user noticing.

2. Steps to create ajax

The principle of Ajax is simply to send an asynchronous request to the server through the XmlHttpRequest object, obtain data from the server, and then use javascript to operate the DOM and update the page. The most critical step in this is to obtain the request data from the server. Creating ajax natively can be divided into the following four steps

1. Create an XMLHttpRequest object

The core of Ajax is the XMLHttpRequest object, which is the key to Ajax implementation. It can send asynchronous requests, accept responses, and execute callbacks. It is done through it

All modern browsers (IE7+, Firefox, Chrome, Safari and Opera) have built-in XMLHttpRequest objects.

Syntax for creating XMLHttpRequest objects:

var xhr = new XMLHttpRequest();

Older versions of Internet Explorer (IE5 and IE6) use ActiveX objects:

var xhr = new ActiveXObject("Microsoft.XMLHTTP");

To cope with all modern browsers, including IE5 and IE6, please check whether the browser supports the XMLHttpRequest object. If supported, an XMLHttpRequest object is created. If it is not supported, create an ActiveXObject:

Compatible with each browser's tool function for creating Ajax

function createRequest (){
 try {
 xhr = new XMLHttpRequest();
 }catch (tryMS){
 try {
 xhr = new ActiveXObject("Msxm12.XMLHTTP");
 } catch (otherMS) {
 try {
 xhr = new ActiveXObject("Microsoft.XMLHTTP");
 }catch (failed) {
 xhr = null;
 }
 }
 }
 return xhr;
}

2. Prepare the request

Initialize the XMLHttpRequest object and accept three parameters :

xhr.open(method,url,async);

The first parameter represents a string representing the request type, and its value can be GET or POST.

GET request:

xhr.open("GET",demo.php?name=tsrot&age=24,true);

POST request:

xhr.open("POST",demo.php,true);

The second parameter is the URL to which the request is to be sent.

The third parameter is true or false, indicating whether the request is issued in asynchronous or synchronous mode. (Default is true, false is generally not recommended)

• false：同步模式发出的请求会暂停所有javascript代码的执行，知道服务器获得响应为止，如果浏览器在连接网络时或者在下载文件时出了故障，页面就会一直挂起。

• true：异步模式发出的请求，请求对象收发数据的同时，浏览器可以继续加载页面，执行其他javascript代码

3、发送请求

xhr.send();

一般情况下，使用Ajax提交的参数多是些简单的字符串，可以直接使用GET方法将要提交的参数写到open方法的url参数中，此时send方法的参数为null或为空。

GET请求：

xhr.open("GET",demo.php?name=tsrot&age=24,true);
xhr.send(null);

POST请求：

如果需要像 HTML 表单那样 POST 数据，请使用 setRequestHeader()来添加 HTTP 头。然后在send()方法中规定您希望发送的数据：

xhr.open("POST",demo.php,true);
xhr.setRequestHeder("Content-Type","application/x-www-form-urlencoded;charset=UTF-8");
xhr.sen

4、处理响应

xhr.onreadystatechange = function(){
 if(xhr.readyState == 4 && xhr.status == 200){
 console.log(xhr.responseText);
 }
}

onreadystatechange ：当处理过程发生变化的时候执行下面的函数

readyState ：ajax处理过程

      0：请求未初始化（还没有调用 open() ）。

      1：请求已经建立，但是还没有发送（还没有调用 send() ）。

      2：请求已发送，正在处理中（通常现在可以从响应中获取内容头）。

      3：请求在处理中；通常响应中已有部分数据可用了，但是服务器还没有完成响应的生成。

      4：响应已完成；您可以获取并使用服务器的响应了。

status属性：

• 200:”OK”
  

• 404: 未找到页面

responseText：获得字符串形式的响应数据

responseXML：获得 XML形式的响应数据

对象转换为JSON格式使用JSON.stringify

json转换为对象格式用JSON.parse()

返回值一般为json字符串，可以用JSON.parse(xhr.responseText)转化为JSON对象

从服务器传回的数据是json格式，这里做一个例子说明，如何利用

1、首先需要从XMLHttpRequest对象取回数据这是一个JSON串，把它转换为真正的JavaScript对象。使用JSON.parse(xhr.responseText)转化为JSON对象

2、遍历得到的数组，向DOM中添加新元素

function example(responseText){
var salep= document.getElementById("sales");
var sales = JSON.parse(responseText);
 for(var i=0;i<sales.length><p><img src="/static/imghwm/default1.png" data-src="https://img.php.cn/upload/article/000/054/025/43128062dd23542fb55c8e5d17b6f416-4.jpg?x-oss-process=image/resize,p_40" class="lazy" alt=""></p>
<p>5、完整例子</p>
<pre class="brush:php;toolbar:false">var xhr = false;
 if(XMLHttpRequest){
 xhr = new XMLHttpRequest();
 }else{
 xhr = new ActiveXObject("Microsoft.XMLHTTP");
};
if(xhr) {//如果xhr创建失败，还是原来的false
 xhr.open("GET","./data.json",true);
 xhr.send();
 xhr.onreadystatechange = function(){
 if(xhr.readyState == 4 && xhr.status == 200){
 console.log(JSON.parse(xhr.responseText).name);
 }
 }
}

data.json

{
 "name":"tsrot",
 "age":24
}

这个过程是一定要记在脑子里的

function ajax(url, success, fail){
 // 1. 创建连接
 var xhr = null;
 xhr = new XMLHttpRequest()
 // 2. 连接服务器
 xhr.open('get', url, true)
 // 3. 发送请求
 xhr.send(null);
 // 4. 接受请求
 xhr.onreadystatechange = function(){
 if(xhr.readyState == 4){
 if(xhr.status == 200){
 success(xhr.responseText);
 } else { // fail
 fail && fail(xhr.status);
 }
 }
 }
}

XMLHttpRequest 在异步请求远程数据时的工作流程

谈谈JSONP

要访问web服务器的数据除了XMLHttpRequest外还有一种方法是JSONP

如果HTML和JavaScript与数据同时在同一个机器上，就可以使用XMLHttpRequest

什么是JSONP？

JSONP(JSON with Padding)是一个非官方的协议，它允许在服务器端集成Script tags返回至客户端，通过javascript callback的形式实现跨域访问（这仅仅是JSONP简单的实现形式）

JSONP有什么用？

由于同源策略的限制，XmlHttpRequest只允许请求当前源（域名、协议、端口）的资源，为了实现跨域请求，可以通过script标签实现跨域请求，然后在服务端输出JSON数据并执行回调函数，从而解决了跨域的数据请求

如何使用JSONP？

在客户端声明回调函数之后，客户端通过script标签向服务器跨域请求数据，然后服务端返回相应的数据并动态执行回调函数
用XMLHttpRequest时，我们得到一个字符串；要用JSON.parse把字符串转化成对象，使用jsonp时，script标志会解析并执行返回的代码，等我们处理数据时，已经是一个JavaScript对象了

简单实例

<meta> 
<script> 
 function jsonpCallback(result) { 
 alert(result.a); 
 alert(result.b); 
 alert(result.c); 
 for(var i in result) { 
 alert(i+":"+result[i]);//循环输出a:1,b:2,etc. 
 } 
 } 
</script> 
<script></script> 
<!--callback参数指示生成JavaScript代码时要使用的函数jsonpcallback-->

注意浏览器的缓存问题

• 在末尾增加一个随机数可避免频繁请求同一个链接出现的缓存问题

• `

三、 jQuery中的Ajax

jQuery中的ajax封装案例

//ajax请求后台数据
var btn = document.getElementsByTagName("input")[0];
btn.onclick = function(){
 
 ajax({//json格式
 type:"post",
 url:"post.php",
 data:"username=poetries&pwd=123456",
 asyn:true,
 success:function(data){
 document.write(data);
 }
 });
}
//封装ajax
function ajax(aJson){
 var ajx = null;
 var type = aJson.type || "get";
 var asyn = aJson.asyn || true;
 var url = aJson.url; // url 接收 传输位置
 var success = aJson.success;// success 接收 传输完成后的回调函数
 var data = aJson.data || '';// data 接收需要附带传输的数据
 
 if(window.XMLHttpRequest){//兼容处理
 ajx = new XMLHttpRequest();//一般浏览器
 }else
 {
 ajx = new ActiveXObject("Microsoft.XMLHTTP");//IE6+
 }
 if (type == "get" && data)
 {
 url +="/?"+data+"&"+Math.random();
 }
 
 //初始化ajax请求
 ajx.open( type , url , asyn );
 //规定传输数据的格式
 ajx.setRequestHeader('content-type','application/x-www-form-urlencoded');
 //发送ajax请求（包括post数据的传输）
 type == "get" ?ajx.send():ajx.send(aJson.data);
 
 //处理请求
 ajx.onreadystatechange = function(aJson){
 
 if(ajx.readState == 4){
 
 if (ajx.status == 200 && ajx.status<p>jQuery中的Ajax的一些方法</p><p>jquery对Ajax操作进行了封装，在jquery中的$.ajax()方法属于最底层的方法，第2层是load() 、$.get() 、$.post();第3层是$.getScript() 、$.getJSON() ，第2层使用频率很高</p><p>load()方法</p><p>load()方法是jquery中最简单和常用的ajax方法，能载入远程HTML代码并插入DOM中 结构为：load(url,[data],[callback])</p><p>使用url参数指定选择符可以加载页面内的某些元素 load方法中url语法：url selector 注意：url和选择器之间有一个空格<br></p><p>传递方式<br></p><p>load()方法的传递方式根据参数data来自动指定，如果没有参数传递，则采用GET方式传递，反之，采用POST<br></p><p>回调参数<br></p><p>必须在加载完成后才执行的操作，该函数有三个参数 分别代表请求返回的内容、请求状态、XMLHttpRequest对象<br>只要请求完成，回调函数就会被触发</p><pre class="brush:php;toolbar:false">$("#testTest").load("test.html",function(responseText,textStatus,XMLHttpRequest){
 //respnoseText 请求返回的内容
 //textStatus 请求状态 ：sucess、error、notmodified、timeout
 //XMLHttpRequest 
})

load方法参数

参数名称	类型	说明
url	String	请求HTML页面的URL地址
data(可选)	Object	发送至服务器的key / value数据
callback(可选)	Function	请求完成时的回调函数，无论是请求成功还是失败

$.get()和$.post()方法

load()方法通常用来从web服务器上获取静态的数据文件。在项目中需要传递一些参数给服务器中的页面，那么可以使用$.get()和$.post()或$.ajax()方法

注意：$.get()和$.post()方法是jquery中的全局函数

$.get()方法

$.get()方法使用GET方式来进行异步请求

结构为：$.get(url,[data],callback,type)

如果服务器返回的内容格式是xml文档，需要在服务器端设置Content-Type类型 代码如下： header("Content-Type:text/xml:charset=utf-8") //php

$.get()方法参数解析

参数	类型	说明
url	String	请求HTML页的地址
data(可选)	Object	发送至服务器的key/ value 数据会作为QueryString附加到请求URL中
callback(可选)	Function	载入成功的回调函数（只有当Response的返回状态是success才调用该方法）
type(可选)	String	服务器返回内容的格式，包括xml、html、script、json、text和_default

$.post() method

It has the same structure and usage as the $.get() method, with the following differences

• GET request The parameters will be passed after Zhang Nai's URL, while the POST request is sent to the web server as the entity content of the Http message. In the ajax request, this difference is invisible to the user

• GET method has a size limit on the transmitted data (usually no more than 2KB), while the amount of data transferred using POST method is much larger than that of GET method (theoretically not limited)

• GET The data requested in this way will be cached by the browser, so others can read the data from the browser's history, such as account number and password. In some cases, the GET method will bring serious security problems, while POST can relatively avoid these problems.

• The data passed by the GET and POST methods cannot be obtained on the server side. same. In PHP, use $_GET[] to get the GET method; use $_POST[] to get the POST method; both methods can use $_REQUEST[] to get it
  

Summary

Use load(), $.get() and $.post() methods to complete some regular Ajax programs. If you need complex Ajax programs, you need to use the $.ajax() method

$.ajax() method

$.ajax() method is the lowest level Ajax implementation of jquery. Its structure is $.ajax(options)

This method has only one parameter, but this object contains the request settings and callback functions required by the $.ajax() method. The parameters exist in key/value, and all parameters are optional

$.ajax() method common parameter analysis

##timeoutNumberSet request timeout (milliseconds) dataTypeStringThe type expected to be returned by the server. The available types are as followsbeforeSendFunctionA function that can modify 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 parametercompleteFunctionCallback function after the request is completed (called when the request succeeds or fails)successFunctionThe callback function called after a successful request has two parameterserrorFunctionFunction called when the request failsglobalBooleanDefault is true. Indicates whether to trigger the global Ajax event. If set to false, it will not be triggered. AjaxStart or AjaxStop can be used to control various Ajax events

Parameter	Type	Description
url	String	(Default is the current page address) The address to send the request
type	String	The request method (POST or GET) defaults to GET
		xml: Returns an XML document, which can be processed with jquery html: Returns plain text HTML information, and the included script tag will also be executed when inserted into the DOM Script: Returns plain text javascript code. Results are not automatically cached unless the cache parameter is set. Note: When making remote requests, all POST requests will be converted to GET requests json: Return JSON data jsonp: JSONP format, when calling a function using jsonp format, for example: myurl?call back=?, jquery will automatically replace the latter one? is the correct function name to execute the callback function text: Returns a plain text string
		Function(XMLHttpRequest){ This;//The options parameters passed when calling this Ajax request }
		Parameters: XMLHttpRequest object and a string describing the successful request type Function(XMLHttpRequest,textStatus){ This;//The options parameters passed when calling this Ajax request }
		(1) Data returned by the server and processed according to the dataType parameter (2) A string describing the status Function(data,textStatus){ //data may be xmlDoc, ``jsonObj, html, text, etc. This;//The options parameters passed when calling this Ajax request }

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