1 Web App Development 2 3 Web App Development qwe function - - PowerPoint PPT Presentation

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1 Web App Development 2 3 Web App Development qwe function - - PowerPoint PPT Presentation

Sep 17, 2022 404 likes •1.13k views

1 Web App Development 2 3 Web App Development qwe function loadDoc(url, callbackFunction) { var xhttp; xhttp=new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) {

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Web App Development

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Web App Development

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qwe

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function loadDoc(url, callbackFunction) { var xhttp; xhttp=new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { callbackFunction(this); } }; url = "https://api.svsu.edu/courses?courseNumber=255"; xhttp.open("GET", url, true); xhttp.send(); } function myFunction(xhttp) { document.getElementById("demo").innerHTML = xhttp.responseText; }

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Web App Development

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AJAX is a developer's dream, because you can:

▪ Read data from a web server - after the page has loaded ▪ Update a web page without reloading the page ▪ Send data to a web server - in the background

Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_first

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<!DOCTYPE html> <html> <body> <div id="demo"> <h2>Let AJAX change this text</h2> <button type="button" onclick="loadDoc()">Change Content</button> </div> </body> </html> The HTML page contains a <div> section and a <button>. The <div> section is used to display information from a server. The <button> calls a function (if it is clicked).

AJAX Example Explained

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The function requests data from a web server and displays it: function loadDoc() { var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("demo").innerHTML = this.responseText; } }; xhttp.open("GET", "ajax_info.txt", true); xhttp.send(); }

AJAX Example Explained Continued

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AJAX = Asynchronous JavaScript And XML. AJAX is not a programming language. AJAX just uses a combination of:

▪ A browser built-in XMLHttpRequest object (to request data from a web server) ▪ JavaScript and HTML DOM (to display or use the data)

AJAX allows web pages to be updated asynchronously by exchanging data with a web server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

What is AJAX?

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AJAX is a misleading name. AJAX applications might use XML to transport data, but it is equally common to transport data as plain text or JSON text.

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How AJAX Works

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  • 1. An event occurs in a web page (the page

is loaded, a button is clicked)

  • 2. An XMLHttpRequest object is created by

JavaScript

  • 3. The XMLHttpRequest object sends a

request to a web server

  • 4. The server processes the request
  • 5. The server sends a response back to the

web page

  • 6. The response is read by JavaScript
  • 7. Proper action (like page update) is

performed by JavaScript

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Web App Development

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The keystone of AJAX is the XMLHttpRequest object.

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All modern browsers support the XMLHttpRequest object. The XMLHttpRequest object can be used to exchange data with a web server behind the scenes. This means that it is possible to update parts of a web page, without reloading the whole page.

The XMLHttpRequest Object

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All modern browsers (Chrome, Firefox, IE7+, Edge, Safari, Opera) have a built-in XMLHttpRequest object. Syntax for creating an XMLHttpRequest object: variable = new XMLHttpRequest(); Example: var xhttp = new XMLHttpRequest(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_xmlhttp

Create an XMLHttpRequest Object

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For security reasons, modern browsers do not allow access across domains. This means that both the web page and the XML file it tries to load, must be located on the same server. The examples on W3Schools all open XML files located on the W3Schools domain. If you want to use the example above on one of your own web pages, the XML files you load must be located on your own server.

Access Across Domains

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Same origin policy

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Old versions of Internet Explorer (5/6) use an ActiveX object instead of the XMLHttpRequest

  • bject:

variable = new ActiveXObject("Microsoft.XMLHTTP"); To handle IE5 and IE6, check if the browser supports the XMLHttpRequest object, or else create an ActiveX object: if (window.XMLHttpRequest) { // code for modern browsers xmlhttp = new XMLHttpRequest(); } else { // code for old IE browsers xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); } Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_xmlhttprequest_ie

Older Browsers (IE5 and IE6)

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XMLHttpRequest Object Methods

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Method Description new XMLHttpRequest() Creates a new XMLHttpRequest object abort() Cancels the current request getAllResponseHeaders() Returns header information getResponseHeader() Returns specific header information

  • pen(method, url, async, user, psw)

Specifies the request method: the request type GET or POST url: the file location async: true (asynchronous) or false (synchronous) user: optional user name psw: optional password send() Sends the request to the server Used for GET requests send(string) Sends the request to the server. Used for POST requests setRequestHeader() Adds a label/value pair to the header to be sent

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XMLHttpRequest Object Properties

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Property Description

  • nreadystatechange

Defines a function to be called when the readyState property changes readyState Holds the status of the XMLHttpRequest. 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready responseText Returns the response data as a string responseXML Returns the response data as XML data status Returns the status-number of a request 200: "OK" 403: "Forbidden" 404: "Not Found" For a complete list go to the Http Messages Reference statusText Returns the status-text (e.g. "OK" or "Not Found")

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The XMLHttpRequest object is used to exchange data with a server.

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To send a request to a server, we use the open() and send() methods of the XMLHttpRequest object: xhttp.open("GET", "ajax_info.txt", true); xhttp.send();

Send a Request To a Server

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Method Description

  • pen(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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GET is simpler and faster than POST, and can be used in most cases. However, always use POST requests when:

▪ A cached file is not an option (update a file or database on the server). ▪ Sending a large amount of data to the server (POST has no size limitations). ▪ Sending user input (which can contain unknown characters), POST is more robust and secure

than GET. GET or POST?

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A simple GET request: xhttp.open("GET", "demo_get.asp", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_get In the example above, you may get a cached result. To avoid this, add a unique ID to the URL: xhttp.open("GET", "demo_get.asp?t=" + Math.random(), true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_get_unique

GET Requests

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If you want to send information with the GET method, add the information to the URL: xhttp.open("GET", "demo_get2.asp?fname=Henry&lname=Ford", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_get2

GET Requests

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A simple POST request: xhttp.open("POST", "demo_post.asp", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_post To POST data like an HTML form, add an HTTP header with setRequestHeader(). Specify the data you want to send in the send() method: xhttp.open("POST", "ajax_test.asp", true); xhttp.setRequestHeader("Content-type", "application/x-www-form- urlencoded"); xhttp.send("fname=Henry&lname=Ford"); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_post2

POST Requests

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POST Requests Continued

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Method Description setRequestHeader(header, value) Adds HTTP headers to the request header: specifies the header name value: specifies the header value

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The url parameter of the open() method, is an address to a file on a server: xhttp.open("GET", "ajax_test.asp", true); The file can be any kind of file, like .txt and .xml, or server scripting files like .asp and .php (which can perform actions on the server before sending the response back).

The url - A File On a Server

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Server requests should be sent asynchronously. The async parameter of the open() method should be set to true: xhttp.open("GET", "ajax_test.asp", true); By sending asynchronously, 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 after the response is ready

Asynchronous - True or False?

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With the XMLHttpRequest object you can define a function to be executed when the request receives an answer. The function is defined in the onreadystatechange property of the XMLHttpResponse

  • bject:

xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("demo").innerHTML = this.responseText; } }; xhttp.open("GET", "ajax_info.txt", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_first You will learn more about onreadystatechange in a later chapter.

The onreadystatechange Property

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To execute a synchronous request, change the third parameter in the open() method to false: xhttp.open("GET", "ajax_info.txt", false); Sometimes async = false are used for quick testing. You will also find synchronous requests in older JavaScript code. Since the code will wait for server completion, there is no need for an

  • nreadystatechange function:

xhttp.open("GET", "ajax_info.txt", false); xhttp.send(); document.getElementById("demo").innerHTML = xhttp.responseText; Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_asyncfalse

Synchronous Request

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Synchronous XMLHttpRequest (async = false) is not recommended because the JavaScript will stop executing until the server response is ready. If the server is busy or slow, the application will hang or stop. Synchronous XMLHttpRequest is in the process of being removed from the web standard, but this process can take many years. Modern developer tools are encouraged to warn about using synchronous requests and may throw an InvalidAccessError exception when it occurs.

Synchronous Request Continued

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The readyState property holds the status of the XMLHttpRequest. The onreadystatechange property defines a function to be executed when the readyState changes. The status property and the statusText property holds the status of the XMLHttpRequest

  • bject.

The onreadystatechange Property

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Property Description

  • nreadystatechange

Defines a function to be called when the readyState property changes readyState Holds the status of the XMLHttpRequest. 0: request not initialized 1: server connection established 2: request received 3: processing request 4: request finished and response is ready status 200: "OK" 403: "Forbidden" 404: "Page not found" For a complete list go to the Http Messages Reference statusText Returns the status-text (e.g. "OK" or "Not Found")

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The onreadystatechange function is called every time the readyState changes. When readyState is 4 and status is 200, the response is ready: function loadDoc() { var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("demo").innerHTML = this.responseText; } }; xhttp.open("GET", "ajax_info.txt", true); xhttp.send(); } Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_first

The onreadystatechange Property Continued

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The onreadystatechange event is triggered four times (1-4), one time for each change in the readyState.

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A callback function is a function passed as a parameter to another function. If you have more than one AJAX task in a website, you should create one function for executing the XMLHttpRequest object, and one callback function for each AJAX task. The function call should contain the URL and what function to call when the response is ready.

Using a Callback Function

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loadDoc("url-1", myFunction1); loadDoc("url-2", myFunction2); function loadDoc(url, cFunction) { var xhttp; xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { cFunction(this); } }; xhttp.open("GET", url, true); xhttp.send(); } function myFunction1(xhttp) { // action goes here } function myFunction2(xhttp) { // action goes here } Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_callback

Using a Callback Function Continued

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Server Response Properties

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Property Description responseText get the response data as a string responseXML get the response data as XML data

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Server Response Methods

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Method Description getResponseHeader() Returns specific header information from the server resource getAllResponseHeaders( ) Returns all the header information from the server resource

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The responseText property returns the server response as a JavaScript string, and you can use it accordingly: document.getElementById("demo").innerHTML = xhttp.responseText; Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_first

The responseText Property

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The XML HttpRequest object has an in-built XML parser. The responseXML property returns the server response as an XML DOM object. Using this property you can parse the response as an XML DOM object: Request the file cd_catalog.xml and parse the response: xmlDoc = xhttp.responseXML; txt = ""; x = xmlDoc.getElementsByTagName("ARTIST"); for (i = 0; i < x.length; i++) { txt += x[i].childNodes[0].nodeValue + "<br>"; } document.getElementById("demo").innerHTML = txt; xhttp.open("GET", "cd_catalog.xml", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_responsexml

The responseXML Property

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You will learn a lot more about XML DOM in the DOM chapters of this tutorial

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The getAllResponseHeaders() method returns all header information from the server response. var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("demo").innerHTML = this.getAllResponseHeaders(); } }; Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_header

The getAllResponseHeaders() Method

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The getResponseHeader() method returns specific header information from the server response. var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("demo").innerHTML = this.getResponseHeader("Last-Modified"); } }; xhttp.open("GET", "ajax_info.txt", true); xhttp.send(); Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_lastmodified

The getResponseHeader() Method

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AJAX can be used for interactive communication with an XML file.

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The following example will demonstrate how a web page can fetch information from an XML file with AJAX: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_xml2

AJAX XML Example

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When a user clicks on the "Get CD info" button above, the loadDoc() function is executed. The loadDoc() function creates an XMLHttpRequest object, adds the function to be executed when the server response is ready, and sends the request off to the server. When the server response is ready, an HTML table is built, nodes (elements) are extracted from the XML file, and it finally updates the element "demo" with the HTML table filled with XML data:

Example Explained

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function loadDoc() { var xhttp = new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { myFunction(this); } }; xhttp.open("GET", "cd_catalog.xml", true); xhttp.send(); } function myFunction(xml) { var i; var xmlDoc = xml.responseXML; var table="<tr><th>Artist</th><th>Title</th></tr>"; var x = xmlDoc.getElementsByTagName("CD"); for (i = 0; i <x.length; i++) { table += "<tr><td>" + x[i].getElementsByTagName("ARTIST")[0].childNodes[0].nodeValue + "</td><td>" + x[i].getElementsByTagName("TITLE")[0].childNodes[0].nodeValue + "</td></tr>"; } document.getElementById("demo").innerHTML = table; }

Example Explained Continued

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The XML file used in the example above looks like this: "cd_catalog.xml".

The XML File

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AJAX is used to create more interactive applications.

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The following example demonstrates how a web page can communicate with a web server while a user types characters in an input field:

AJAX PHP Example

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In the example above, when a user types a character in the input field, a function called "showHint()" is executed. The function is triggered by the onkeyup event. Here is the HTML code:

Example Explained

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Here is the HTML code: <html> <head> <script> function showHint(str) { if (str.length == 0) { document.getElementById("txtHint").innerHTML = ""; return; } else { var xmlhttp = new XMLHttpRequest(); xmlhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("txtHint").innerHTML = this.responseText; } }; xmlhttp.open("GET", "gethint.php?q=" + str, true); xmlhttp.send(); } } </script> </head> <body> <p><b>Start typing a name in the input field below:</b></p> <form> First name: <input type="text" onkeyup="showHint(this.value)"> </form> <p>Suggestions: <span id="txtHint"></span></p> </body> </html> Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_suggest_php

Example Explained Continued

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Code explanation: First, check if the input field is empty (str.length == 0). If it is, clear the content of the txtHint placeholder and exit the function. However, if the input field is not empty, do the following:

▪ Create an XMLHttpRequest object ▪ Create the function to be executed when the server response is ready ▪ Send the request off to a PHP file (gethint.php) on the server ▪ Notice that q parameter is added gethint.php?q="+str ▪ The str variable holds the content of the input field

Example Explained Continued

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The PHP file checks an array of names, and returns the corresponding name(s) to the browser: <?php // Array with names $a[] = "Anna"; $a[] = "Brittany"; $a[] = "Cinderella"; $a[] = "Diana"; $a[] = "Eva"; $a[] = "Fiona"; $a[] = "Gunda"; $a[] = "Hege"; $a[] = "Inga"; $a[] = "Johanna"; $a[] = "Kitty"; $a[] = "Linda"; $a[] = "Nina"; $a[] = "Ophelia"; $a[] = "Petunia"; $a[] = "Amanda"; $a[] = "Raquel"; $a[] = "Cindy"; $a[] = "Doris"; $a[] = "Eve"; $a[] = "Evita"; $a[] = "Sunniva"; $a[] = "Tove"; $a[] = "Unni"; $a[] = "Violet"; $a[] = "Liza"; $a[] = "Elizabeth"; $a[] = "Ellen"; $a[] = "Wenche"; $a[] = "Vicky"; // get the q parameter from URL $q = $_REQUEST["q"]; $hint = ""; // lookup all hints from array if $q is different from "" if ($q !== "") { $q = strtolower($q); $len=strlen($q); foreach($a as $name) { if (stristr($q, substr($name, 0, $len))) { if ($hint === "") { $hint = $name; } else { $hint .= ", $name"; } } } } // Output "no suggestion" if no hint was found or output correct values echo $hint === "" ? "no suggestion" : $hint; ?>

The PHP File - "gethint.php"

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AJAX is used to create more interactive applications.

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The following example will demonstrate how a web page can communicate with a web server while a user type characters in an input field:

AJAX ASP Example

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In the example above, when a user types a character in the input field, a function called "showHint()" is executed. The function is triggered by the onkeyup event.

Example Explained

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Here is the HTML code: <html> <head> <script> function showHint(str) { if (str.length == 0) { document.getElementById("txtHint").innerHTML = ""; return; } else { var xmlhttp = new XMLHttpRequest(); xmlhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { document.getElementById("txtHint").innerHTML = this.responseText; } }; xmlhttp.open("GET", "gethint.asp?q=" + str, true); xmlhttp.send(); } } </script> </head> <body> <p><b>Start typing a name in the input field below:</b></p> <form> First name: <input type="text" onkeyup="showHint(this.value)"> </form> <p>Suggestions: <span id="txtHint"></span></p> </body> </html> Try it Yourself: https://www.w3schools.com/js/tryit.asp?filename=tryjs_ajax_suggest_asp

Example Explained Continued

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Code explanation: First, check if the input field is empty (str.length == 0). If it is, clear the content of the txtHint placeholder and exit the function. However, if the input field is not empty, do the following:

▪ Create an XMLHttpRequest object ▪ Create the function to be executed when the server response is ready ▪ Send the request off to an ASP file (gethint.asp) on the server ▪ Notice that q parameter is added gethint.asp?q="+str ▪ The str variable holds the content of the input field

Example Explained Continued

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The ASP file checks an array of names, and returns the corresponding name(s) to the browser:

▪

<% response.expires=-1 dim a(30) 'Fill up array with names a(1)="Anna" a(2)="Brittany" a(3)="Cinderella" a(4)="Diana" a(5)="Eva" a(6)="Fiona" a(7)="Gunda" a(8)="Hege" a(9)="Inga" a(10)="Johanna" a(11)="Kitty" a(12)="Linda" a(13)="Nina" a(14)="Ophelia" a(15)="Petunia" a(16)="Amanda" a(17)="Raquel" a(18)="Cindy" a(19)="Doris" a(20)="Eve" a(21)="Evita" a(22)="Sunniva" a(23)="Tove" a(24)="Unni" a(25)="Violet" a(26)="Liza" a(27)="Elizabeth" a(28)="Ellen" a(29)="Wenche" a(30)="Vicky"

▪

'get the q parameter from URL q=ucase(request.querystring("q")) 'lookup all hints from array if length of q>0 if len(q)>0 then hint="" for i=1 to 30 if q=ucase(mid(a(i),1,len(q))) then if hint="" then hint=a(i) else hint=hint & " , " & a(i) end if end if next end if 'Output "no suggestion" if no hint were found 'or output the correct values if hint="" then response.write("no suggestion") else response.write(hint) end if %>

The ASP File - "gethint.asp"

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qwe

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function loadDoc(url, callbackFunction) { var xhttp; xhttp=new XMLHttpRequest(); xhttp.onreadystatechange = function() { if (this.readyState == 4 && this.status == 200) { callbackFunction(this); } }; url = "https://api.svsu.edu/courses?courseNumber=255"; xhttp.open("GET", url, true); xhttp.send(); } function myFunction(xhttp) { document.getElementById("demo").innerHTML = xhttp.responseText; }

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