learning about javascript (…and its little buddy, jquery!)
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Learning About JavaScript…and its little buddy, JQuery!
GWU Libraries ● 26 June 2012Julie Meloni // @jcmeloni // jcmeloni@gmail.com
Today’s General Outline
•Understanding JavaScript Basics
•About the DOM (Document Object Model)
•Where JQuery Fits in and How it Works
Understanding JavaScript Basics
The Very Very Basics
•Developed in 1995 – it’s no spring chicken•No relation to the Java programming
language•An interpreted rather than a compiled
language▫Interpreted by the web browser software
•98% of web traffic is JavaScript-enabled▫2% made up of old screenreader software
and ultra-security-conscious people
Practical Uses
•Improving navigation •Offering dynamic text display•Creating special effects in response to
user•As the basis for remote scripting (the J in
AJAX)•Offering more interactivity than static
HTML▫But less dynamism than a server-side
language
How it Looks in the Source Code<script type="text/javascript"><!-- Hide the script from old browsersdocument.write(document.lastModified);// Stop hiding the script --></script>
Might display: 06/25/2012 08:07:51
How it Looks in the Source Code v2<!DOCTYPE html><html> <head> <title>JS Test</title> </head>
<body> <h1>JS Test</h1><p style=“color: red”><script type="text/javascript"><!-- Hide the script from old browsersdocument.write(document.lastModified);// Stop hiding the script --></script></p> </body></html>
Moving Into Programming Basics•Like other programming languages,
JavaScript has:▫Variables (& Arrays)▫Operators▫Flow Control▫Objects
About JavaScript Variables
• A variable is a bucket that holds one piece of information.
• Examples:
• var string_variable = “The Library”;
• var numeric_variable = 4;
• var myname = “Julie”;
About JavaScript Arrays
• An array is a type of variable (or bucket) that holds many pieces of information.
• Example:
• rainbow = new array(“red”, “orange”, “yellow”, “green”, “blue”, “indigo”, “violet”);
rainbow[0] holds “red”
rainbow[1] holds “orange”
About JavaScript Operators• Arithmetic • +, -, *, / (add, subtract, multiply, divide)
• Assignment• = (“Assign the value of 4 to the variable called a”)
var a = 4;• += (“Add the value of 5 to the variable that
already holds 4”) var a += 5; // a now holds 9
• -= (“Subtract the value of 3 to the variable that already holds 4”)
var a -= 3; // a now holds 1
About JavaScript Operators• Comparison• == (“when I compare the value in variable a to the
value in variable be, that comparison is true”) a == b
• != (“when I compare the value in variable a to the value in variable be, that comparison is not true”)
• a != b• >, >= (“the value of variable a is greater than (or
greater than or equal to) the value of variable b”) a > b
• <, <= (“the value of variable a is less than (or less than or equal to) the value of variable b”)
a < b
About JavaScript Operators
• Concatenation• + (“this”+ “is a test”= “thisisatest”)
• Logical• && (and)
• || (or)
• ! (not)
About JavaScript Flow Control• if
if (something is true) {do something here
}• if ... else if ... else
if (something is true) {do something here
} else if (something is true) {do something here
} else {do something here
}
About JavaScript Flow Control• switch
switch (something) {case “blue”:
do something herebreak;
case “apple”:do something else herebreak;
case “fire truck”:do something else herebreak;
default: do something else herebreak;
}
About JavaScript Flow Control
• whilewhile (something is true) {
do something here}
• forfor (something is true) {
do something here}
About JavaScript Objects
•Objects can store multiple pieces of data, but the internal structure of the data is different than that of an array.• The items of data stored in an object are called the properties of the
object. People objects in an address book might include a name, an address, and a
telephone number (Bob.address, Bob.phone, etc)• Objects can use methods.
Methods are built-in functions that work with the object's data, e.g. Bob.display()
• JavaScript supports three kinds of objects:• Built-in to the JavaScript language. • Custom objects you create. • DOM (Document Object Model) -- components of the browser and the
current HTML document.
About the DOM (The Document Object Model)
What is the DOM?
•The DOM is the invisible structure of all HTML documents▫The overall container object is called the
document. Any container within the document that has
an ID is referenced by that ID. For example, if you have a <div> with an ID
called wrapper, then in the DOM that element is referenced by document.wrapper
But it actually starts before that…
•The DOM is not part of JavaScript or any other programming language -- it's an API built in to the browser. • At the top of the browser object hierarchy
is the window object.• Also access the history object and the
location object• Inside a window is a document.• window.document.header-image<img id=“header-image” src=“some.jpg”/>
So what about document?
•The document object has several properties, such as:• document.URL • document.title • document.lastModified • document.cookie • document.images• …and more!
And within document?• The document object contains…containers (also called
nodes).
<!DOCTYPE html> <html> <head> <title>A Simple HTML Document</title> </head> <body> <h1>This is a Level-1 Heading.</h1> <p>This is a simple paragraph.</p></body></html>
Nodes Have Properties Too!
<h1>This is a Level-1 Heading.</h1>
• nodeName is the name of the node, e.g. h1• innerHTML is the text within the node, e.g.
“This is a Level-1 Heading”
So…events?•JavaScript can be used to detect events and
react to them• Events include clicked buttons, mouse pointers moving, etc.
•The script that you use to detect and respond to an event is called an event handler. • You don't need the <script> tag to define an event handler;
you can add an event handler attribute to an individual HTML tag.
<a href="http://www.google.com/" onmouseover="alert('You moved!');">
I’m a link.</a><a href="" onmouseover="DoIt();">Move Me</a>
A Few Event Examples
•Mouse• onmouseover• onmouseout• onmouseup• onmousedown• onclick• ondblclick
•Keyboard• onkeydown• onkeyup
•Form Elements• onblur • onchange • onsubmit
Where JQuery Fits in and How it Works
Why JQuery (or any library?)• Using a third-party library• enables you to implement cross-browser scripting and
sophisticated UI elements without being a JavaScript expert.• enables you to avoid reinventing the wheel for common tasks• Execute code when the DOM is ready• Collect elements• Modify display• Listen for events and enact upon them• Execute AJAX requests• …and more!
Loading JQuery• Must be present in each calling page.• You can download and host your own, or use a remotely hosted
version.<!DOCTYPE html> <html> <head> <title>JQuery</title> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" type="text/javascript"></script> </head> <body> <!-- more html --> </body> </html>
Then What?• jQuery has at its heart sophisticated, cross-browser methods for
selection of page elements. • Selectors used to obtain elements; based on simple CSS-like selector
styles.• To get an element that has an ID of someElement, use:
$("#someElement")• To reference a collection of elements that use the someClass class name, use:
$(".someClass")
• You can manipulate HTML and CSS properties using built-in methods.• To append the phrase "powered by jQuery" to all paragraph elements, for
example, we would simply write: $("p").append(" powered by jQuery");
• To then change the background color of those same elements, we can manipulate their CSS properties directly: $("p").css("background-color","yellow");
• To hide all elements having in class hideMe, use: $(".hideMe").hide();
Moving Forward
•Understanding what you want to do will help you find the JQuery examples to do it.▫Everything is based on interactions (and
events)▫Everything is rooted in things you want to
do with HTML and CSS Even if it’s AJAX
Getting the Document Ready<!DOCTYPE html> <html> <head> <title>JQuery</title> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" type="text/javascript"></script> </head> <body> <script type="text/javascript">$(document).ready(function(){ // Your code here });</script> </body> </html>
Getting the Document Ready – Alert!<!DOCTYPE html> <html> <head> <title>JQuery</title> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" type="text/javascript"></script> </head> <body>
<script type="text/javascript">$(document).ready(function(){ $("a").click(function(event){
alert("Hi!");event.preventDefault();
});});</script><a href="http://www.google.com">Go to Google</a>
</body> </html>
Fading In – An Effect Example<!DOCTYPE html> <html> <head> <title>JQuery</title> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js" type="text/javascript"></script> </head> <body>
<div id="clickme">Click here to fade in</div>
<img id="logo" src="logo.jpg" height="112" width="241" alt="logo" style="display:none" /><script type="text/javascript">
$('#clickme').click(function() { $('#logo').fadeIn('slow', function() {}); });
</script> </body> </html>
Where to Go From Here?
•ANYWHERE• Hopefully right to the JQuery API
documentation at http://api.jquery.com
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