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Introduction to JavaScript

This talk introduces the JavaScript language.

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What is JavaScript?

Browsers have limited functionality

Text, images, tables, frames

JavaScript allows for interactivity
Initially Developed as a Client-Side scripting language

Create Dynamic Web Content in the Browser

Browser/page manipulation

Reacting to user actions

A type of programming language

Easy to learn
Developed by Netscape
Now a standard exists – �www.ecma-international.org/publications/�standards/ECMA-262.HTM

When the web was conceived, browsers were limited to text and image - later tables and frames. The extent to which they provided interactivity with the user was very limited. JavaScript was developed by Netscape as a simple programming language (often referred to as a scripting language). It is easy to learn and small sections of JavaScript can be added to a web page rather than needing to develop complicated programs. It was specially designed for web page interaction and manipulating the web browser and page elements. It is often used to respond to user actions such as mouse clicks.

Although developed by Netscape, and other variants exist, such as Jscript from Microsoft, a standard has been developed by the European Computer Manufacturers Association. It is known as ECMAScript, using the standard ECMA262, which can be found fully documented at the address on the slide.

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Java and JavaScript

Netscape Mozilla working with Sun (creators of Java) in the 1990’s�
They were developing a web-scripting language to be embedded in HTML and executed by the browser – originally called LiveScript

Changed name to JavaScript to foster the Sun partnership.

They share some common syntax and approaches but very different languages

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JavaScript Allows Interactivity

Improve appearance

Especially graphics
Visual feedback

Site navigation
Perform calculations
Validation of input

Examples should be shown which demonstrate different uses of JavaScript. For example, try the calculator example at the address shown.

JavaScript can provide interactivity and/or enhance pages in several ways. It is particularly good at manipulating browser elements and images to improve page presentation and navigation.

As a programming language it can handle quite complex calculations and control the behaviour of embedded content including images.

A useful function is input validation - checking form responses BEFORE the form is sent for processing to a server. In this way it can be quicker and more user friendly, providing immediate feedback to the user.

JavaScript can be combined with other technologies such as Java applets or plug-ins, but this is beyond the scope of this presentation.

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How Does It Work?

Embedded within HTML page

View source

Executes on client

Fast, no connection needed once loaded

Simple programming statements combined with HTML tags
Interpreted (not compiled)

No special tools required

JavaScript is embedded/included within HTML. You can often see JavaScript in the source of a web page or it is provided for information on the page as with the calculator example.

JavaScript is mainly used as a client-side language - it downloads with the web page. Once the page has downloaded and is on the users' machine, it is actually the web browser which then interprets the JavaScript instructions. JavaScript pages run quickly, you are not relying on an internet connection to a web server. Short pieces of JavaScript can be combined with HTML without the need to develop a fully blown program.

There are two types of computer language, compiled and interpreted. To write or edit a compiled language requires a special piece of software called a compiler. JavaScript belongs to the other category, called interpreted. In the case of JavaScript, this interpretation is done by the browser software at run-time. Because JavaScript is interpreted, this means that no special tools are required to write or edit JavaScript, just a normal text editor. JavaScript web pages can be platform independent i.e. they will run on different browsers and computers (as long as the browser is JavaScript enabled). If you see a JavaScript web page that you like, you may be able to take that JavaScript and use it for your own purposes. (Remember to acknowledge the original author!)

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JavaScript Statements

<html>

<body>

document.write('This is my first →

JavaScript Page');

</script>

</body>

</html>

Note the symbol for �line continuation

JavaScript can be contained either in the header section of an HTML page or in the body. This JavaScript statement is shown as a pure JavaScript statement within SCRIPT tags.

Notice that there is no HTML in the body of this page at all. (Demonstrate what this JavaScript looks like in a web browser).

This statement writes a line of text on a web page.

The command document.write is a standard function in JavaScript to write text to the page. The following is a more technical explanation for background information only:

document.write is derived from the JavaScript object model (not covered in detail here). It works on the principle that all document and browser elements have an object name (document, window, image etc) and can each has various properties that can be manipulated. The object hierarchy means that individual elements can be uniquely identified i.e. document.myform.mytext would refer to the text entry named mytext within the form called myform within the current page (document).

The arrow symbol '→' is used in these slides and in the workbook to indicate where a JavaScript statement should be typed on one line without a break. A line break in the wrong place will stop JavaScript from working.e.g.

document.write('This is my first →

JavaScript Page');

should actually be typed:

document.write('This is my first JavaScript Page');

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JavaScript Statements

<html>

<body>

document.write('<h1>This is my first →

JavaScript Page</h1>');

</script>

</body>

</html>

HTML written

inside JavaScript

Document.write is the JS “Method”

This example demonstrates that anything included within the quotes in the document.write statement is printed to the screen, and this includes HTML tags. The <h1> tag is delivered to the browser along with the text, and the browser would interpret it as a normal HTML file, displaying the text in the Heading 1 style.

IMPORTANT NOTE:

This example shows a JavaScript statement in the <body> of the web page.

It is possible to include JavaScript statements in the <head> section of a web page but care must be taken that they do not try to access items that don't exist until the page has loaded (e.g. form elements, links, images). The web browser parses (reads through and executes) any script commands as it displays the page.

In most cases it is common sense that dictates where a statement should be placed.

If, in the above example, document.write was placed in the <head> of the page, the text "This is my first JavaScript Page" would appear in the <head> of the finished page – this would be incorrect – although modern browsers will let you get away with it!

In some circumstances you may wish to use document.write in the <head> - for example to dynamically generate <meta> or <title> tags. Such uses are not considered here.

JavaScript functions are typically defined in the <head> section of a web page as they do not normally execute until they have been triggered elsewhere. The use of functions in JavaScript is covered in the Netskills Training Module: "Further JavaScript (Enhancing JavaScript with Functions and Events)"

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JavaScript Statements

<html>

<body>

<p>

<br />

<a href="myfile.html"

onMouseover="window.alert('Hello');">

My Page</A>

</p>

</body>

</html>

JavaScript written

inside HTML

An Event

Compare this example with the previous one. This time the JavaScript is written inside the HTML tags and there are no <script> tags.

In this case if the browser is JavaScript-enabled it will process the commands when it needs to. If the browser doesn't understand JavaScript it will ignore the extra code (it should see it as an HTML attribute that it cannot process and therefore ignores, although very old browsers my throw an error)

This example demonstrates an HTML hyperlink, but notice the JavaScript enclosed within the <a href.. tag of the second link.

onMouseOver is referring to an event. That is, this JavaScript will happen in response to something that the user does e.g click a button, or in this case, when they move the mouse over the link (this will not happen if you move your mouse over the first link!).

window.alert will display what is called an alert box on the screen containing the text specified, in this case, "hello".

The first link will behave normally.

(See separate Netskills Training Module for more details on Functions and Events in JavaScript.)

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Example Statements

window.prompt('Enter your name:','');

</script>

<form>

</form>

Another event

Note quotes: " and '

This example shows two separate statements. The first is some pure JavaScript so must be contained within <script> tags. It displays a pop up box with the message "Enter your name" and a space to type in text. This occurs immediately when the page loads.

The second statement is within an HTML form. The <input> tag includes some JavaScript within it. The tag creates a form element (a button) and associates with it some JavaScript. The JavaScript responds to the mouse click event (onClick) on the button by producing a pop up alert box with the text 'Hello' in. This only occurs when the user triggers the event by clicking on the button.

JavaScript makes use of both single and double quotes. They are used in this example to have a text string 'Hello' contained within another string which defines the code to be carried out in response to the onClick event. It is important that the quotes should match - if they don't, your JavaScript will not work!

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JavaScript window.alert Method

Alert Box

An alert box is often used if you want to make sure information comes through to the user.
When an alert box pops up, the user will have to click "OK" to proceed.

Syntax

window.alert(‘sometext’);

The window.alert() method can be written without the window prefix.

alert(‘I am an alert box!’);

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onerror Event

Execute a JavaScript if an error occurs when loading an image:

So – useful in XSS….

If you reference an image file that does not exist, it will run the error function/method