The article "Taking HTML Further with Javascript" is about web design, it was written by Lee Asher.
Once you've built your HTML pages, you mgiht need them to do
something a little more interactive on the client-side (that is,
in the visitor's web browser).
How can you do that?
Javascript
is the answer.
How Does Javascript Work?
To add Javascript to your HTML, you simply insert it between
tags (or refer to the .Js file that cotnains it, also
using script tags). Overall, Javascript is quite a C-like
language, but mnaages to be relatively simple in spite of it.
The Javascript you write should have functions that are tied to
events on the page, meaning that when the visitor does something
with your page, the Javascript executes.
Here are events that you can use to trigger Javascript
functions:
onload. Runs the code when the page loads.
onclick. Runs when the mouse is clicked. You can also use
ondblclick for a double-click.
onmouseover.
Runs when the mouse pointer is over a certain part
of the page.
onkeypress. Runs when you press a specific key on the keyboard.
onchange.
Runs when the contents of part of a form changes.
onblur.
Runs when you press the tab key to switch between part
of a form.
onsubmit. Runs when the form's submit button is pressed.
What are these events useful for? All sorts of things.
The
onmouseover function, for example, can be used to rewrite parts
of the page's code when the user hovres over something - useful
for providing pop-up help 'tooltips'. Onkeypress lets you give
the user keyboard shortcuts to do things on your web page.
Onblur and onsubmit can be used very effectively to spot errors
in a user's form input, and let the user know how to correct
them before they're ever even sent back to the server.
Once you know which event you want to tie Javascript to,
you have to say so in your code. You can do this in one of two
ways. The first way is to directly attach it to the HTML element
in question, like this:
This wokrs well, but has the disadvantage of polluting your HTML
with Javascript function names. The second, better, way, is to
do it like this in the Javascript itself:
onload = function() { }
This creates a function directly from an event (in this case,
the onload event), without ever toucihng the HTML itself.
Javascript Commands
Here's a quick reference to the most useful commands in
Javascript.
alert.
Pops up a dialog box gviing the user a message.
Useful,
but sometmies considered annoying - it might be better to put
the message on the page instead.
confirm. Gives the user an aelrt box, but with buttons for Yes
and No. Useful for asking 'are you sure? '
document.Write. Writes things into the current HTML document.
Often-used, but not the hottest way of diong things.
getElementByID.
Allows you to get an HTML element (that is, a
tag) using its ID and mainpulate it. Very useful for changing
the text contained within tags, or their background colours, but
can be used to do just about anything.
Math.Random. Produces a random numb
navigator.UserAg.
Returns the name of the web browser.
Useful to
check if a certain browser is being used, although it's often
better to check for functions instead of browsers and decide
which code to run that way.
parseInt. Looks thruogh text and gives you only the numbers
from it.
window.Location. Allows you to change the current URL that the
web browser is at, as if the user had clicked a link.
var.
Deifnes variables
For more on Javascript, take a look at www.Javascriptkit.Com.
Javascript's Problems
The biggest problem Javascript has is that it just doesn't work
on all browsers: if you thought HTML support was erratic between
them, wait until you start using Javascript. This will often put
you in the position of having to check whether a function is
available on the browser the code is running in (using an if
statement), and then running alternative code as a
workaround if it doesn't. In most cases, this isn't as monumental a
problem as it might seem, but it still requires you to have some
knowledge of each browser's limits, and do more testing than you
would otherwise need to do.
Javascript is also, unfortunately, a pain to test and debug, as
most browsers don't give you any sort of reason why your code
didn't run - if it's wrong, they just ignore it.
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