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JavaScript, Fourth Edition

JavaScript, Fourth Edition. Chapter 12 Updating Web Pages with AJAX. Objectives. Study AJAX concepts Learn about HTTP Use AJAX to request and receive server data. JavaScript, Fourth Edition. 2. 2. Introduction to AJAX. Asynchronous JavaScript and XML (AJAX)

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JavaScript, Fourth Edition

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  1. JavaScript, Fourth Edition Chapter 12 Updating Web Pages with AJAX

  2. Objectives • Study AJAX concepts • Learn about HTTP • Use AJAX to request and receive server data JavaScript, Fourth Edition JavaScript, Fourth Edition 2 2

  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 JavaScript, Fourth Edition

  4. 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 JavaScript, Fourth Edition 4

  5. Introduction to AJAX (continued) 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 JavaScript, Fourth Edition 5

  6. Introduction to AJAX (continued) JavaScript, Fourth Edition 6

  7. Introduction to AJAX (continued) JavaScript, Fourth Edition 7

  8. Introduction to AJAX (continued) JavaScript, Fourth Edition 8

  9. Introduction to AJAX (continued) 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 JavaScript, Fourth Edition 9

  10. Introduction to AJAX (continued) JavaScript, Fourth Edition 10

  11. 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 JavaScript, Fourth Edition 11

  12. 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 JavaScript, Fourth Edition 12

  13. Accessing Content on a Separate Domain (continued) JavaScript, Fourth Edition 13

  14. Accessing Content on a Separate Domain (continued) JavaScript, Fourth Edition 14

  15. 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 JavaScript, Fourth Edition 15

  16. 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 JavaScript, Fourth Edition 16

  17. 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 JavaScript, Fourth Edition 17

  18. 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 JavaScript, Fourth Edition 18

  19. 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 JavaScript, Fourth Edition 19

  20. 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 JavaScript, Fourth Edition 20

  21. 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 JavaScript, Fourth Edition 21

  22. 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 JavaScript, Fourth Edition 22

  23. Sending HTTP Requests (continued) JavaScript, Fourth Edition 23

  24. Sending HTTP Requests (continued) JavaScript, Fourth Edition 24

  25. 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 JavaScript, Fourth Edition

  26. Receiving HTTP Responses (continued) Status codes format (continued) 5xx: (server error)— Request cannot be fulfilled due to a server error JavaScript, Fourth Edition 26

  27. Receiving HTTP Responses (continued) 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 JavaScript, Fourth Edition 27

  28. Receiving HTTP Responses (continued) Example Create a PHP script that returns the RSS feeds for the selected news agency in the top stories program JavaScript, Fourth Edition 28

  29. Receiving HTTP Responses (continued) JavaScript, Fourth Edition 29

  30. 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 JavaScript, Fourth Edition 30

  31. Requesting Server Data (continued) JavaScript, Fourth Edition 31

  32. Requesting Server Data (continued) JavaScript, Fourth Edition 32

  33. 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 JavaScript, Fourth Edition 33

  34. 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 JavaScript, Fourth Edition 34

  35. 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 JavaScript, Fourth Edition 35

  36. 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 JavaScript, Fourth Edition 36

  37. 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 JavaScript, Fourth Edition 37

  38. 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 JavaScript, Fourth Edition 38

  39. 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 JavaScript, Fourth Edition 39

  40. Receiving Synchronous Responses (continued) JavaScript, Fourth Edition 40

  41. Receiving Synchronous Responses (continued) JavaScript, Fourth Edition 41

  42. 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 JavaScript, Fourth Edition 42

  43. 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 JavaScript, Fourth Edition 43

  44. 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 JavaScript, Fourth Edition 44

  45. 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 JavaScript, Fourth Edition

  46. 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 JavaScript, Fourth Edition

  47. 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 47

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