JavaScript Notes
JavaScript Origins
JavaScript
was released by Netscape and Sun Microsystems in 1995.
However,
JavaScript is not the same thing as Java.
What is JavaScript
• It is a programming language.
• It is an interpreted language.
• It is object-based programming.
• It is widely used and supported
• It is accessible to the beginner.
Uses of JavaScript
• Use it to add multimedia elements
With
JavaScript you can show, hide, change, resize images, and
create
image rollovers. You can create scrolling text across the status
bar.
• Create pages dynamically
Based
on the user's choices, the date, or other external data, JavaScript
can
produce pages that are customized to the user.
• Interact with the user
It
can do some processing of forms and can validate user input when
the
user submits the form.
Writing JavaScript
JavaScript
code is typically embedded in the HTML, to be interpreted and
run
by the client's browser. Here are some tips to remember when writing
JavaScript
commands.
• JavaScript code is case sensitive
• White space between words and tabs are ignored
• Line breaks are ignored except within a statement
• JavaScript statements end with a semi- colon ;
The SCRIPT Tag
The
<SCRIPT> tag alerts a browser that JavaScript code follows. It is
typically
embedded in the HTML.
<SCRIPT language =
"JavaScript">
statements
</SCRIPT>
SCRIPT Example
• Open "script_tag.html" in a browser.
• View the Source
• Put the cursor after <! – Enter code below and enter the following:
<SCRIPT language =
"JavaScript">
alert("Welcome to the script tag
test page.")
</SCRIPT>
• Save the changes by choosing Save from the File menu.
• Then Refresh the browser by clicking the Refresh or Reload
button.
Implementing JavaScript
There
are three ways to add JavaScript commands to your Web Pages.
• Embedding code
• Inline code
• External file
External File
You
can use the SRC attribute of the <SCRIPT> tag to call JavaScript code
from
an external text file. This is useful if you have a lot of code or you
want
to run it from several pages, because any number of pages can call the
same
external JavaScript file. The text file itself contains no HTML tags. It
is
call by the following tag:
<SCRIPT
SRC="filename.js">
</SCRIPT>
External Example
• Open "external.html" in a browser
• View the Source
• Put the cursor after <! – Enter code here and enter:
<SCRIPT language =
"JavaScript" SRC = "external.js">
</SCRIPT>
• Save the changes and Refresh the browser.
2 Programming Basics
Programmers
use variables to store values. A variable can hold several types
of
data. In JavaScript you don't have to declare a variable's data type before
using
it. Any variable can hold any JavaScript data type, including:
• String data
• Numbers
• Boolean values (T/F)
Variable Names
There
are rules and conventions in naming variables in any programming
language.
It is good practice to use descriptive names for variables. The
following
are the JavaScript rules:
• The variable name must start with a letter or an underscore.
firstName or _myName
• You can use numbers in a variable name, but not as the first
character.
name01 or tuition$
• You can't use space to separate characters.
userName not user Name
• Capitalize the first letter of every word except the first
salesTax or userFirstName
Variables
• To declare variables, use the keyword var and the
variable name:
var userName
• To assign values to variables, add an equal sign and the value:
var userName = "Smith"
var price = 100
Functions
With
functions, you can give a name to a whole block of code, allowing you
to
reference it from anywhere in your program. JavaScript has built-in
functions
for several predefined operations. Here are three some functions.
• alert("message")
• confirm("message")
• prompt("message")
Function Example
• Open "functions. html" and View the Source
• Put the cursor after " // add code here " and enter:
• var userName
var willDoSurvey
userName = prompt("Enter your
name", "")
alert("Thank you, " +
userName)
• Save the changes and Refresh the page
User-Defined Functions
With
user-defined functions, you can name a block of code and call it when
you
need it. You define a function in the HEAD section of a web page. It is
defined
with the function keyword, followed by the function name and any
arguments.
function functionName(argument)
{
statements
}
