JavaScript For Beginners(reprint)_Basic knowledge-JS Tutorial-php.cn

Why do we place JavaScript code inside comment fields  ?
It's for ensuring that the Script is not displayed by old browsers that do not support JavaScript. This is optional, but considered good practice. The LANGUAGE attribute also is optional, but recommended. You may specify a particular version of JavaScript:

< script language ="JavaScript1.2" >

You can use another attribute SRC to include an external file containing JavaScript code:

< script language ="JavaScript" src ="hello.js" > script >

For example, shown below is the code of the external file hello.js :

document.write("Hello World!")

The external file is simply a text file containing JavaScript code with the file name extension ".js".

Note:

Including an external file only functions reliably across platforms n the version 4 browsers.
The code can't include tags , or you will get an error message.

write and writeln

In order to output text in JavaScript you must use write() or writeln(). Here's an example:

< HTML >
< HEAD >
< TITLE > Welcome to my site TITLE > HEAD >
< BODY >
< SCRIPT LANGUAGE ="JAVASCRIPT" >

SCRIPT >
BODY >
HTML >

Note: the document object write is in lowercase as JavaScript is case sensitive. The difference between write and writeln is: write just outputs a text, writeln outputs the text and a line break.

Document object

The document object is one of the most important objects of JavaScript. Shown below is a very simple JavaScript code:

document.write("Hi there.")

In this code, document is the object. write is the method of this object. Let's have a look at some of the other methods that the document object possesses.

lastModified

You can always include the last update date on your page by using the following code:

< script language ="JavaScript" >
document.write( " This page created by John N. Last update: " + document.lastModified);
script >

All you need to do here is use the lastModified property of the document. Notice that we used <font color="#0000ff">+</font> to put together This page created by John N. Last update: and document.write.

bgColor and fgColor

Lets try playing around with bgColor and fgColor:

< script >
document.bgColor = " black "
document.fgColor = " #336699 "
script >

Message Box

alert

There are three message boxes: alert, confirm, and prompt. Let's look at the first one:

< body >
< script >
window.alert( " Welcome to my site! " )
script >
body >

You can put whatever you want inside the quotation marks.

confirm

An example for confirm box:

window.confirm("Are you sure you want to quit?")

prompt

Prompt box is used to allow a user to enter something according the promotion:

window.prompt("please enter user name")

In all our examples above, we wrote the box methods as window.alert(). Actually, we could simply write the following instead as:

alert()
confirm()
prompt()

Variables and Conditions

Let's see an example:

< script >
var x = window.confirm( " Are you sure you want to quit " )

if (x)
window.alert( " Thank you. " )
else
window.alert( " Good choice. " )
script >

There are several concepts that we should know. First of all, var x = is a variable declaration. If you want to create a variable, you must declare the variable using the var statement. x will get the result, namely, true or false . Then we use a condition statement if else to give the script the ability to choose between two paths, depending on this result (condition for the following action). If the result is true (the user clicked "ok"), "Thank you" appears in the window box. If the result is false (the user clicked "cancel"), "Good choice" appears in the window box instead. So we can make more complex boxes using var, if and those basic methods.

< script >
var y = window.prompt( " please enter your name " )
window.alert(y)
script >

Another example:

< html >< head >
< script >
var x = confirm( " Are you sure you want to quit? " )
if ( ! x)
window.location = " http://www.yahoo.com "
script >
head >
< body >
Welcome to my website!.
body > html >

If you click "cancel", it will take you to yahoo, and clicking ok will continue with the loading of the current page "Welcome to my website!". Note: if(!x) means: if click "cancel". In JavaScript, the exclamation mark !means: "none".

Function

Functions are chunks of code.Let's create a simple function:

function test()
{
document.write("Hello can you see me?")
}

Note that if only this were within your <script> </script> tags, you will not see "Hello can you see me?" on your screen because functions are not executed by themselves until you call upon them. So we should do something:

function test()
{
document.write("Hello can you see me?")
}
test()

Last line test() calls the function, now you will see the words "Hello can you see me?".

Event handler

What are event handlers? They can be considered as triggers that execute JavaScript when something happens, such as click or move your mouse over a link, submit a form etc.

onClick

onClick handlers execute something only when users click on buttons, links, etc. Let's see an example:

< script >
function ss()
{
alert( " Thank you! " )
}
script >
< form >
< input type ="button" value ="Click here" onclick ="ss()" >
form >

The function ss() is invoked when the user clicks the button. Note: Event handlers are not added inside the

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