javascript, fourth edition chapter 12 updating web pages with ajax

JavaScript, Fourth Edition

Chapter 12Updating Web Pages with AJAX

JavaScript, Fourth Edition 2JavaScript, Fourth Edition 22

Objectives

• Study AJAX concepts• Learn about HTTP• Use AJAX to request and receive server data

JavaScript, Fourth Edition 3

Introduction to AJAX• Asynchronous JavaScript and XML (AJAX)

– Refers to a combination of technologies– Allows Web pages displayed on a client computer to

quickly interact and exchange data• With a Web server without reloading the entire Web page

• AJAX primarily relies on JavaScript and HTTP requests– To exchange data between a client computer and a

Web server

• XML is often the format used for exchanging data


Introduction to AJAX (continued)

• Other technologies that comprise AJAX– XHTML, CSS, and the Document Object Model

(DOM)

• XMLHttpRequest object– Uses HTTP to exchange data between a client

computer and a Web server

– Can be used to request and receive data without reloading a Web page



• Combining XMLHttpRequest with DHTML– You can update and modify individual portions of

your Web page• With data received from a Web server

• Google Suggest Web site– www.google.com/webhp?complete=1

– One of the first commercial Web sites to implement an AJAX application



• Example– Create an AJAX application that retrieves the

top stories from a selected news agency using RSS feeds

• RSS (for RDF Site Summary, Rich Site Summary, or Really Simple Syndication)– XML format that allows Web sites to publish

content that can be read by other Web sites


Understanding AJAX’s Limitations

• The data you request must be located on the Web server where your JavaScript program is running

• You can use a server-side script as a proxy to access data from another domain

• Proxy– Refers to someone or something that acts or

performs a request for another thing or person


Accessing Content on a Separate Domain

• Web service, or XML Web service– Software component that resides on a Web server– Does not contain any sort of graphical user interface

or even a command-line interface– Simply provides services and data in the form of

methods and properties• It is up to the client to provide an implementation for a

program that calls a Web service

• Example– AJAX example that displays streaming stock quote

information from Yahoo! Finance


Accessing Content on a Separate Domain (continued)


Running AJAX from a Web Server

• You must open your AJAX files from a Web server – With the HTTP protocol (http://)

• Apache HTTP Server– Most popular Web server software used on the

Internet

• Second most popular Web server– Microsoft Internet Information Services (IIS)

• Example– Open the stock quotes Web page from your Web

server


Overview of Creating an AJAX Script

• Steps to create an AJAX script– Instantiate an XMLHttpRequest object for the

Web browser where the script will run

– Use the XMLHttpRequest object to send a request to the server

– Read and process the data returned from the server


Working with HTTP

• Request– Process of asking for a Web page from a Web

server

• Response– Web server’s reply

• Every Web page is identified by a unique address called the Uniform Resource Locator, or URL

• HTTP client– Refers to the application, usually a Web browser,

which makes the request


Working with HTTP (continued)

• HTTP server– Another name for a Web server– Refers to a computer that receives HTTP requests and

returns responses to HTTP clients

• Host– Refers to a computer system that is being accessed by a

remote computer

• HTTP is a component of Transmission Control Protocol/Internet Protocol (TCP/IP)

• W3C and Internet Engineering Task Force jointly develop HTTP


Understanding HTTP Messages

• HTTP messages– HTTP client requests and server responses

• HTTP client opens a connection to the server and submits a request message

• Web server then returns a response message that is appropriate to the type of request

• Headers– Define information about the request or

response message and about the contents of the message body


Understanding HTTP Messages (continued)

• Cache-Control header– Specifies how a Web browser should cache any

server content it receives

• Caching– Refers to the temporary storage of data for

faster access

– Web browser will attempt to locate any necessary data in its cache

• Before making a request from a Web server

– It goes against the reason for using AJAX


Understanding HTTP Messages (continued)

• A blank line always follows the last header line– Optionally, a message body can follow the blank

line in the messages

• Most common types of HTTP requests– GET and POST

• Other HTTP request– HEAD, DELETE, OPTIONS, PUT, and TRACE


Sending HTTP Requests

• GET method– Used for standard Web page requests

– Can have a query string or form data appended to the URL

• POST request– Similar to a GET request except that any

submitted data is included in the message body• Immediately following the blank line after the last

header


Sending HTTP Requests (continued)


Receiving HTTP Responses

• HTTP response messages– Take the same format as request messages– Return the protocol and version of the HTTP server

• Along with a status code and descriptive text

• Status codes format– 1xx: (informational)—Request received– 2xx: (success)—Request successful– 3xx: (redirection)—Request cannot be completed without

further action– 4xx: (client error)—Request cannot be fulfilled due to a

client error


Receiving HTTP Responses (continued)

• Status codes format (continued)– 5xx: (server error)— Request cannot be fulfilled

due to a server error



• Zero or more response headers follow the status line

• Response returned from a server can be much more involved– Than the original request that generated it



• Example– Create a PHP script that returns the RSS feeds

for the selected news agency in the top stories program


Requesting Server Data

• XMLHttpRequest object– Key to turning your JavaScript script into AJAX

programs

– Allows you to use JavaScript and HTTP to exchange data between a Web browser and a Web server


Requesting Server Data (continued)


Instantiating an XMLHttpRequest Object

• For Mozilla-based browsers and Internet Explorer 7– Use the XMLHttpRequest constructor

• For older versions of Internet Explorer– You must instantiate the XMLHttpRequest object as

an ActiveX object

• ActiveX– Technology that allows programming objects to be

easily reused• With any programming language that supports

Microsoft’s Component Object Model


Instantiating an XMLHttpRequest Object (continued)

• Component Object Model (COM)– Architecture for cross-platform development of

client/server applications

• Most JavaScript programmers use a series of nested try...catch statements– To instantiate an XMLHttpRequest object according to

the Web browser that runs the script

• Opening and closing HTTP connections takes up a lot of computer memory and processing time– HTTP/1.1 automatically keeps the client-server

connection open unless it is specifically closed


Instantiating an XMLHttpRequest Object (continued)

• You can make your AJAX programs faster by reusing an instantiated XMLHttpRequest object

• Example– Add code to the top stories Web page that

instantiates an XMLHttpRequest object


Opening and Sending a Request

• Use the open() method with the instantiated XMLHttpRequest object– To specify the request method (such as GET or POST) and URL

• open() method accepts three optional arguments– User name, password, and the async argument

• abort() method– Used to cancel any existing HTTP requests

before beginning a new one


Opening and Sending a Request (continued)

• send() method– Submit the request to the server

– Accepts a single argument containing the message body

• Example– Add a function that instantiates, opens, and

submits an XMLHttpRequest object


Receiving Server Data

• responseXML property– Contains the HTTP response as an XML document

• responseText property– Contains the HTTP response as a text string

• In the XML DOM, each XML element is referred to as a node

• childNodes[] array– Returns an array of child nodes for an element

• nodeValue property– Sets and returns the value of a node


Receiving Synchronous Responses

• Synchronous request– Stops the processing of the JavaScript code until a

response is returned from the server

• Check the value of the XMLHttpRequest object’s status property– Ensure that the response was received successfully

• Example– Modify the top stories Web page so it sends and

receives synchronous requests and responses using RSS feeds


Receiving Synchronous Responses (continued)



• Synchronous responses are easier to handle• Drawback

– Script will not continue processing until the response is received

• You should use asynchronous requests with the send() method


Receiving Asynchronous Responses

• Asynchronous request– Allows JavaScript to continue processing while it

waits for a server response

• Create an asynchronous request– Pass a value of true as the third argument of the open() method

• Or omit the argument altogether

• Receive a response– Use the XMLHttpRequest object’s readyState

property and onreadystatechange event


Receiving Asynchronous Responses (continued)

• Value assigned to the readyState property is updated automatically– According to the current statement of the HTTP

request

• If property is assigned a value of 4– The response is finished loading

• Example– Modify the top stories Web page so it sends and

receives asynchronous requests and responses


Summary

• “Asynchronous JavaScript and XML” or “AJAX”• The XMLHttpRequest object uses HTTP to

exchange data between a client computer and a Web server

• RSS (RDF Site Summary or Rich Site Summary) is an XML format that allows Web sites to publish content that can be read by other Web sites

• You cannot use the XMLHttpRequest object to directly access content on another domain’s server


Summary (continued)

• You must open AJAX files from a Web server with the HTTP protocol (http://)

• Hypertext Transfer Protocol (HTTP)• HTTP client requests and server responses are both

known as HTTP messages• Use the methods and properties of an instantiated XMLHttpRequest object with JavaScript to build and send request messages

• First step for using AJAX to exchange data between an HTTP client and a Web server is to instantiate an XMLHttpRequest object


Summary (continued)

• To improve performance, you should call the abort() method of the XMLHttpRequest object

• Use the send() method with the instantiated XMLHttpRequest object to submit the request to the server

• A synchronous request stops the processing of the JavaScript code until a response is returned

• Asynchronous request allows JavaScript to continue processing while it waits for a server response

javascript, fourth edition chapter 12 updating web pages with ajax

Documents