Javascript Tutorial

JavaScript Basics

In this lesson, we'll learn about the core functionality of the JavaScript language.

Lesson Goals

• To work with the HTML DOM.
• To follow JavaScript syntax rules.
• To write JavaScript inline.
• To write JavaScript in script blocks.
• To create and link to external JavaScript files.
• To work with JavaScript objects, methods, and properties.
• To reference HTML elements with JavaScript.
• To use event handlers.

Lesson Activities

1. The Name "JavaScript"
2. JavaScript and the HTML DOM
3. JavaScript Syntax
4. Accessing Elements
5. Where Is JavaScript Code Written?
6. javascript: Pseudo-Protocol
7. JavaScript Objects, Methods and Properties
8. Event Handlers

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The Name "JavaScript"

In this course, we refer to the language we are learning as JavaScript, which is what it is usually called. However, JavaScript was invented by Netscape Communications and is now owned by Oracle Corporation. Microsoft calls its version of the language JScript. JavaScript and JScript are both implementations of EcmaScript, but everyone still refers to the language as JavaScript.

The HTML DOM

The HTML Document Object Model (DOM) is the browser's view of an HTML page as an object hierarchy, starting with the browser window itself and moving deeper into the page, including all of the elements on the page and their attributes. Below is a simplified version of the HTML DOM.

As shown, the top-level object is window. The document object is a child of window and all the objects (i.e, elements or nodes) that appear on the page (e.g, forms, links, images, tables, etc.) are descendants of the document object. These objects can have children of their own. For example, form objects generally have several child objects, including textboxes, radio buttons, and select menus.

JavaScript Syntax

Basic Rules

1. JavaScript statements end with semi-colons.
2. JavaScript is case sensitive.
3. JavaScript has two forms of comments:
   • Single-line comments begin with a double slash (//).
   • Multi-line comments begin with "/*" and end with "*/".

Syntax

// This is a single-line comment

/*
	This is
	a multi-line
	comment.
*/

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Accessing Elements

Dot Notation

In JavaScript, elements (and other objects) can be referenced using dot notation, starting with the highest-level object (i.e, window). Objects can be referred to by name or id or by their position on the page. For example, if there is a form on the page named "loginform", using dot notation you could refer to the form as follows:

window.document.loginform

Assuming that loginform is the first form on the page, you could also refer to this way:

window.document.forms[0]

A document can have multiple form elements as children. The number in the square brackets ([]) indicates the specific form in question. In programming speak, every document object contains a collection of forms. The length of the collection could be zero (meaning there are no forms on the page) or greater. In JavaScript, collections (and arrays) are zero-based, meaning that the first form on the page is referenced with the number zero (0) as shown in the syntax example above.

Square Bracket Notation

Objects can also be referenced using square bracket notation as shown below:

window['document']['loginform']

// and 

window['document']['forms'][0]

Dot notation and square bracket notation are completely interchangeable. Dot notation is much more common; however, as we will see later in the course, there are times when it is more convenient to use square bracket notation.

The Implicit window Object

The window object is always the implicit top-level object and therefore does not have to be included in references to objects. For example, window.document.forms[0] can be shortened to document.forms[0].

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Where Is JavaScript Code Written?

JavaScript code can be written inline (e.g, within HTML tags called event handlers), in script blocks, and in external JavaScript files. The page below shows examples of all three.

Code Sample:

<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>JavaScript Page</title>
<link href="style.css" rel="stylesheet" type="text/css">
<script type="text/javascript">
	window.alert("The page is loading");
</script>
</head>
<body>
<p>
	<span onclick="document.bgColor='red';">Red</span> |
	<span onclick="document.bgColor='white';">White</span>
</p>
<script type="text/javascript" src="script-1.js"></script>
</body>
</html>

Code Sample:

document.write("<p>Hello, there!</p>");

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javascript: Pseudo-Protocol

Try this:

1. Open your browser.
2. In the location bar, type in javascript:alert("Hello world!"); like so:
3. Press Enter.

You should get an alert reading "Hello world!". The javascript: prefix is called a pseudo-protocol, because it mimics the protocol syntax (e.g, like http:, ftp:, and mailto:). It provides an easy way to test JavaScript on a page. It can also be used in links as in the demo below:

Code Sample:

<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>Pseudo-Protocol</title>
</head>
<body>
<p><a href="javascript:alert('Hello world!');">Say hello</a></p>
</body>
</html>

This is generally considered a bad practice. The preferred way to handle this is to use the href attribute to provide an alternative page in case JavaScript is not enabled. You could provide a generic page with a "JavaScript is required" message or, even better, you could provide a page that accomplishes the same task as the alert as in the following the demo:

Code Sample:

<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>Better than Pseudo-Protocol</title>
</head>
<body>
<p><a href="hello-world.html" onclick="alert('Hello world!'); return false;">Say hello</a></p>
</body>
</html>

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JavaScript Objects, Methods and Properties

JavaScript is used to manipulate or get information about objects in the HTML DOM. Objects in an HTML page have methods (actions, such as opening a new window or submitting a form) and properties (attributes or qualities, such as color and size).

To illustrate objects, methods and properties, we will look at the code in JavaScriptBasics/Demos/javascript-2.html, a slightly modified version of JavaScriptBasics/Demos/javascript-1.html, which we looked at earlier, and at the code in JavaScriptBasics/Demos/script-2.js.

Code Sample:

<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8">
<title>JavaScript Page</title>
<link href="style.css" rel="stylesheet" type="text/css">
<script type="text/javascript">
	//Pop up an alert
	window.alert("The page is loading");
</script>
</head>
<body>
<p>
	<span onclick="document.bgColor = 'red';">Red</span> |
	<span onclick="document.bgColor = 'white';">White</span> |
	<span onclick="document.bgColor = 'green';">Green</span> |
	<span onclick="document.bgColor = 'blue';">Blue</span>
</p>
<script type="text/javascript" src="script-2.js"></script>
</body>
</html>

Code Sample:

/*
This script simply outputs
	"Hello, there!"
to the browser.
*/
document.write("<p>Hello, there!</p>");

Methods

Methods are the verbs of JavaScript. They cause things to happen.

window.alert()

HTML pages are read and processed from top to bottom. The JavaScript code in the initial script block at the top of JavaScriptBasics/Demos/javascript-2.html calls the alert() method of the window object. When the browser reads that line of code, it will pop up an alert box and will not continue processing the page until the user presses the OK button. Once the user presses the button, the alert box disappears and the rest of the page loads.

document.write()

The write() method of the document object is used to write out code to the page as it loads. In JavaScriptBasics/Demos/script-2.js, it simply writes out "Hello, there!"; however, it is more often used to write out dynamic data, such as the date and time on the user's machine.

Arguments

Methods can take zero or more arguments separated by commas.

Syntax

object.method(argument1, argument2);

The alert() and write() methods shown in the example above each take only one argument: the message to show or the HTML to write out to the browser.

Properties

Properties are the adjectives of JavaScript. They describe qualities of objects and, in some cases are writable (can be changed dynamically).

document.bgColor

The bgColor property of the document object is read-write. Looking back at JavaScriptBasics/Demos/javascript-2.html, the four span elements use the onclick event handler to catch click events. When the user clicks on a span, JavaScript is used to change the value of the bgColor property to a new color.

The Implicit window Object

The window object is always the implicit top-level object and therefore does not have to be included in references to objects. For example, window.document.write() can be shortened to document.write(). Likewise, window.alert() can be shortened to just alert().

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Event Handlers

In JavaScriptBasics/Demos/javascript-2.html, we used the onclick event handler to call JavaScript code that changed the background color of the page. Event handlers are attributes that force an element to "listen" for a specific event to occur. Event handlers all begin with the letters "on". The table below lists the HTML event handlers with descriptions.

HTML Event Handlers

Event Handler	Elements Supported	Description
onblur	a, area, button, input, label, select, textarea	the element lost the focus
onchange	input, select, textarea	the element value was changed
onclick	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer button was clicked
ondblclick	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer button was double clicked
onfocus	a, area, button, input, label, select, textarea	the element received the focus
onkeydown	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a key was pressed down
onkeypress	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a key was pressed and released
onkeyup	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a key was released
onload	frameset	all the frames have been loaded
onload	body	the document has been loaded
onmousedown	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer button was pressed down
onmousemove	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer was moved within
onmouseout	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer was moved away
onmouseover	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer was moved onto
onmouseup	All elements except applet, base, basefont, bdo, br, font, frame, frameset, head, html, iframe, isindex, meta, param, script, style, title	a pointer button was released
onreset	form	the form was reset
onselect	input, textarea	some text was selected
onsubmit	form	the form was submitted
onunload	frameset	all the frames have been removed
onunload	body	the document has been removed

For a full reference of HTML event handler attributes, see the list of attributes and the elements they apply to on the W3C web site.

The getElementById() Method

A very common way to reference HTML elements is by their ID using the getElementById() method of the document object as shown in the example below.

getElementById()

Description	Syntax	Parameter
Used to accesses/manipulate the first element with the specified id.	document.getElementById("id")	The "id" parameter is required. This refers to the id of the HTML element you want to access/manipulate

Code Sample:

---- C O D E   O M I T T E D ----

<p>
	<span onclick="document.getElementById('divRed').bgColor = 'red';">
		Red</span> |
	<span onclick="document.getElementById('divOrange').bgColor = 'orange';">
		Orange</span> |
	<span onclick="document.getElementById('divGreen').bgColor = 'green';">
		Green</span> |
	<span onclick="document.getElementById('divBlue').bgColor = 'blue';">
		Blue</span>
</p>
<table>
	<tr id="divRed"><td>Red</td></tr>
	<tr id="divOrange"><td>Orange</td></tr>
	<tr id="divGreen"><td>Green</td></tr>
	<tr id="divBlue"><td>Blue</td></tr>
</table>
</body>
</html>

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Continue to the next lesson on Variables, Arrays and Operators »