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Editor to share with you what the key knowledge that must be mastered in the AJAX application, I believe that most people do not know much about it, so share this article for your reference, I hope you will learn a lot after reading this article, let's go to know it!

What is AJAX?

Is the acronym of Asynchronous Javascript And XML, it is not a new language, but a comprehensive use of existing technology. Its essence is to communicate with the server asynchronously on the basis of HTTP protocol.

The concept of asynchronous?

It means that when a program is executed, it will not block the execution of other programs, and its manifestation is that the execution order of the program does not depend on the writing order of the program itself, on the contrary, it is synchronous.

The effect of asynchronism?

Its advantage is that it does not block the execution of the program, so as to improve the overall execution efficiency.

The core of AJAX?

XMLHTTPRequest, abbreviated XHR,Xml Extensible markup language, HTTP Hypertext transfer Protocol, Request request. The XMLHttpRequest object can partially update a web page without submitting the entire page to the server. The XMLHttpRequest object provides full access to the HTTP protocol, including the ability to make POST and HEAD requests as well as normal GET requests. XMLHttpRequest can return the response from the Web server synchronously or asynchronously, and the content can be returned as text or an DOM document. It can receive text documents in any form and is a key function of AJAX's Web application architecture.

Earlier, it was mentioned that XMLHTTPRequest can receive any form of document, so we have to mention two data formats commonly used in network transmission.

Two commonly used data formats xml and JSON?

Xml:

Definition of xml: extensible markup language, which is a markup language for tagging electronic documents to make them structural.

The specification of xml:

1. There must be a root element

2. No spaces, no numbers or. Beginning, case-sensitive

3. Non-cross nesting

4. Attribute double quotation marks (browser automatically corrected to double quotation marks)

5. Entities are used for special symbols.

6. Comments are the same as HTML

Although complex data can be described and transmitted, its parsing is too complex and large, so implementation developers have rarely used it.

A brief description of the steps for using xml:

Because xml is a DOM object, you can directly DOM the method

1. Var xml=xhr.responseXML; acquires XML (the interface given by PHP)

2. Var items=xml.querySelector ('item'); get the document structure of xml

3. String concatenation

Initialize var html=''

Traversing for () {get data var item=item [I],} stitching html+=' tags + data'

4. Render document.querySelector ('tbody') .innerHTHML=html

JSON:

JSON definition: JavaScript Object Notation, another lightweight text data exchange format, independent of language.

The specification of JSON:

1. Data is in the name / value pair

2. Data is separated by commas (the last key / value pair cannot be comma)

3. Curly braces save object square brackets save array

4. Use double quotation marks

Cross-language resolution of JSON:

When JSON data is transmitted in different languages, the type is a string, and different languages have their own parsing methods, which need to be parsed before they can be read.

1. PHP parsing method

Array to json character json_encode- > $json_array = json_encode ($array)

Json characters into an array json_decode- > $array_json = json_decode ($json_array)

2. Javascript parsing method

String transfer object parse- > var jsonObj=JSON.parse (jsonStr)

Object to the string stringify- > var jsonStr=JSON.stringify (jsonObj)

Tips:JSON compatible processing reference json2.js

Event listener onreadystatechange in XMLHttpRequest?

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