Introduction to JSON & Ajax

C O M P I L E D B Y : S E B L E N .
D E P A R T M E N T O F C O M P U T E R S C .
A D D I S A B A B A U N I V E R S I T Y
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• JSON is a syntax for storing and exchanging data
• JSON is an easier-to-use alternative to XML
• JSON is language independent
• JSON uses JavaScript syntax, but the JSON format is text
only, just like XML

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JSON
XML

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Similarities
• Both JSON and XML are
hierarchical (values within values)
• Both JSON and XML can be
parsed by any programming
language
• Both JSON and XML can be
fetched with an XMLHttpRequest
Differences
• JSON doesn't use end tag
• JSON is shorter
• JSON is quicker to read and
write
• JSON can use arrays
• XML has to be parsed with
an XML parser. JSON can be
parsed by a standard
JavaScript function

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• JSON syntax is derived from JavaScript object notation
• Example:
• Javascript Object
• JSON Object
• JSON Text/String
• JSON Array

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• Data is in name/value pairs
• name and value are separated by “:” and field name is double
quoted
• Value can be
• A number (integer or floating point)
• A string (in double quotes)
• A Boolean (true or false)
• An array (in square brackets)
• An object (in curly braces)
• null

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• The JavaScript function JSON.parse(text) can be used to
convert a JSON text into a JavaScript object:
• The JavaScript function JSON.stringify(object) can be used to
convert JavaScript object to JSON String

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• A common use of JSON is to exchange data between the
client and a web server
• Objects in PHP can be converted into JSON by using the PHP
function json_encode()

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Asynchronous JavaScript and XML
• AJAX allows web pages to be updated asynchronously by
exchanging small amounts of data with the server behind the scenes
• With Ajax you can:-
• Update a web page without reloading the page
• Request data from a server - after the page has loaded
• Receive data from a server - after the page has loaded
• Send data to a server - in the background
• Examples of applications using AJAX:
• Google Maps, Gmail, YouTube, and Facebook.

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• Many of the tasks performed on the server are very time consuming.
Before AJAX, retrieving data from server behind the scenes could
cause the application to hang or stop
• With AJAX, the JavaScript does not have to wait for the server
response, but can instead:
• execute other scripts while waiting for server response
• deal with the response when the response ready

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XMLHttpRequest Object
• The keystone of AJAX is the XMLHttpRequest object
• The XMLHttpRequest object is used to exchange data with a
server behind the scenes
• Syntax for creating an XMLHttpRequest object:
• var xhttp = new XMLHttpRequest();

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Send a Request to a Server
• To send a request to a server, the open() and send() methods
of the XMLHttpRequest object are used:
Method Description
open(method, url,
async)
Specifies the type of request
method: the type of request: GET or POST
url: the server (file) location
async: true (asynchronous) or false
(synchronous)
send() Sends the request to the server (used for GET)
send(string) Sends the request to the server (used for POST)

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Send a Request to a Server …
• xhttp.open("GET", "ajax_info.txt", true);
xhttp.send();
• xhttp.open("GET", “reg.php?fname=Henry&lname=Ford", true);
xhttp.send();
• xhttp.open("POST", "demo_post.php", true);
xhttp.send();
• To POST form, add an HTTP header with setRequestHeader()
xhttp.open("POST", "ajax_test.asp", true);
xhttp.setRequestHeader("Content-type", "application/x-www-form-
urlencoded");
xhttp.send("fname=Henry&lname=Ford");

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AJAX - Server Response
• The response from a server, can be accessed through the
responseText or responseXML property of the
XMLHttpRequest object
• responseText - get the response data as a string
• responseXML - get the response data as XML data
• document.getElementById("demo").innerHTML =
xhttp.responseText;

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• The readyState property holds the status of the
XMLHttpRequest: Changes from 0 to 4
• 0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready
• The onreadystatechange event is triggered every time the
readyState changes.

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• onreadystatechange event: specify what will happen when
the server response is ready to be processed
• When readyState is 4 and status is 200, the response is ready

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• If you have more than one AJAX task on your
website, you should create ONE standard function
for creating the XMLHttpRequest object, and call
this for each AJAX task

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• jQuery provides several methods for AJAX
functionality.
• With the jQuery AJAX methods, you can request
text, HTML, XML, or JSON from a remote server
using both HTTP Get and HTTP Post

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• The load() method loads data from a server and puts the
returned data into the selected element
• Syntax
• $(selector).load(URL,data,callback);
• URL: required parameter, specifies the URL you wish to load.
• data: optional, specifies a set of querystring key/value pairs to
send along with the request.
• callback: optional, is the name of a function to be executed after
the load() method is completed.
• $("#div1").load("demo_test.txt");

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• specifies a callback function to run when the load() method is
completed
• The callback function can have different parameters:
• responseTxt - contains the resulting content if the call
succeeds
• statusTxt - contains the status of the call
• xhr - contains the XMLHttpRequest object

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• The jQuery get() and post() methods are used to request data
from the server with an HTTP GET or POST request
• GET is used for just getting data from the server
• Note: The GET method may return cached data
• POST can also be used to get some data from the server.
However, the POST method NEVER caches data, and is often
used to send data along with the request.

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• The $.get() method requests data from the server with an HTTP
GET request.
• Syntax
• $.get(URL,callback);
• URL : required parameter, specifies the URL you wish to
request.
• callback: optional parameter , is the name of a function to be
executed if the request succeeds. Has two parameters:
• Data: holds the content of the page requested
• Status: holds the status of the request

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• The $.post() method requests data from the server with an HTTP
POST request.
• Syntax
• $.get(URL,data, callback);
• URL : required parameter, specifies the URL you wish to
request.
• data: optional, specifies some data to send along with the request
• callback: optional parameter , is the name of a function to be
executed if the request succeeds. Has two parameters:
• Data: holds the content of the page requested
• Status: holds the status of the request

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Introduction to JSON & Ajax

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Introduction to JSON & Ajax