Contents
[Contents] [Tutorial]
[Object
Hierrachy] [Language]
[Expressions]
[Statements]
Introduction
Programming languages have come a long way since the early days
of computing. JavaScript is an interpreted
language. The program which executes JavaScript, reads each
line, scanning it for JavaScript, if any is found it is analysed,
then executed.
JavaScript is inserted into HTML documents by enclosing a JavaScript
section between the <SCRIPT LANGUAGE="JavaScript"> and
</SCRIPT> tags. Where JavaScript sections are placed depends
on what it is meant to do:
- If the result is required at a particular location on a page,
the section is placed where the result is wanted
- If the section contains code which other parts of the document
make use of (functions
for example), the code should be placed at the begining of
the document (between the <HEAD> </HEAD> tags
is a good place). Trying to make calls to sections which appear
further down the document can fail, because those sections may
not have been read yet.
[Contents] [Tutorial]
[Object
Hierrachy] [Language]
[Expressions]
[Statements]
Language Composition
A programming language allows symbols to be manipulated (expressions)
and evaluated (operators).
The object hierarchy lists objects, such as a window or a document.
There also fundamental things a programming language must deal with,
such as numbers, text and true/false scenarios. These symbols are
essential if useful programs are to be developed.
It is necessary to find out how expressions are written in JavaScript
before trying to understand the language statements. This is because
the language needs expressions to perform its tasks.
The followinfg link
explains JavaScript expressions and related subjects.
A programming language allows a sequence of statements (commands)
to be defined.
A programming language allows a designer (programmer to non programmers)
to lay out a set of steps, or rules, needed to do something. At
its simplest level it can be just one command, but not much can
be achieved at this level.
This is a simple example, which displays today's date:
<SCRIPT LANGUAGE="JavaScript"> document.write(Date())
</SCRIPT>
If we use two commands, the first command is executed then
the second one. This happens faster than the eye can see. It is
important to understand that an ordered sequence of commands we
write is executed in the order we designed. We can control a sequence
of events which is repeated by anyone who happens to run the program,
or in the case of JavaScript, by anyone who looks at our Web page.
This example has two commands to display the date this page was
last modified, and today's date:
<SCRIPT LANGUAGE="JavaScript">
document.write(document.lastModified)
document.write(" ", Date())
</SCRIPT>
JavaScript Statement Rules
A single JavaScript statement can span one or more lines. If
many statements are put on the same line, they must be seperated
by a semi-colon ;
JavaScript is case sensitive, this means that an upper case character
is not the same as a lower case character: www is not the same
as wWw or WWW.
The following link
explains the JavaScript statements.
[Contents] [Tutorial]
[Object
Hierrachy] [Language]
[Expressions]
[Statements]
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