JavaScript For Beginners(reprint)_Basic knowledge

注：我对原文进行了编辑，对一些词汇标注颜色，方便阅读。本来准备翻译，但是觉得文章简单易懂，而且原文写得很好，所以就不献丑了。希望对JavaScript初学者能有所帮助。你可以跟着作者一起做那些示例代码，等读完文章的时候，你就可以掌握JavaScript的基本操作了，你会发现其实这一切很容易。

Contents
Embedding and including
write and writeln
Document object
Message box
Function
Event handler
Form
Link
Date
Window
Frame

Embedding and including

Let's first see a simple example:  

html >
head >
title > This is a JavaScript example title >
script  language ="JavaScript" >

script >
head >
body >  Hi, man!  body >
html >

Usually, JavaScript code starts with the tag and ends with the tag . The code placed between and . Sometimes, people embed the code in the tags:

html >
head > head >
body >
script >
.. //  The code embedded in the  tags.
script >
body >
html >

Why do we place JavaScript code inside comment fields <!-- and //--> ?
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:

1. Including an external file only functions reliably across platforms n the version 4 browsers.
2. The code can't include tags  and , 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 <script></script> tags, but rather, inside the html tags.

onLoad

The onload event handler is used to call the execution of JavaScript after loading:   

body  onload ="ss()" >
frameset  onload ="ss()" >
img  src ="whatever.gif"  onload ="ss()" >

onMouseover,onMouseout

These handlers are used exclusively with links.

a  href ="#"  onMouseOver ="document.write('Hi, nice to see you!" > Over Here! a >
a  href ="#"  onMouseOut ="alert('Good try!')" > Get Out Here! a >

onUnload

onUnload executes JavaScript while someone leaves the page. For example to thank users. 

body  onunload ="alert('Thank you for visiting us. See you soon')" >

Handle multiple actions

How do you have an event handler call multiple functions/statements? That's simple. You just need to embed the functions inside the event handler as usual, but separate each of them using a semicolon: 

form >
input  type ="button"  value ="Click here!"  onClick ="alert('Thanks for visiting my site!');window.location='http://www.yahoo.com'" >
form >

 

---

Form

Let's say you have a form like this:   

form  name ="aa" >
input  type ="text"  size ="10"  value =""  name ="bb" > br >
input  type ="button" value ="Click Here" onclick ="alert(document.aa.bb.value)" >
form >

Notice that we gave the names to the form and the element. So JavaScript can gain access to them.

onBlur

If you want to get information from users and want to check each element (ie: user name, password, email) individually, and alert the user to correct the wrong input before moving on, you can use onBlur. Let's see how onBlur works: 

html >
head >
script >
function  emailchk()
{
var  x = document.feedback.email.value
if  (x.indexOf( " @ " ) ==- 1 )
{
    alert( " It seems you entered an invalid email address. " )
    document.feedback.email.focus()
}
}
script >
head >

body >
form
name ="feedback" >
Email: input  type ="text"  size ="20"  name ="email"
onblur ="emailchk()" > br >
Comment:  textarea  name ="comment"  rows ="2"  cols ="20" > textarea > br >
input  type ="submit"  value ="Submit" >
form >
body >
html >

If you enter an email address without the @, you'll get an alert asking you to re-enter the data . What is: x.indexOf("@")==-1? This is a method that JavaScript can search every character within a string and look for what we want. If it finds it will return the position of the char within the string. If it doesn't, it will return -1. Therefore, x.indexOf("@")==-1basically means: "if the string doesn't include @, then: 

alert("It seems you entered an invalid email address.")
document.feedback.email.focus()

What's focus() ? This is a method of the text box, which basically forces the cursor to be at the specified text box. onsubmitUnlike onblur, onsubmit handler is inserted inside the

 tag, and not inside any one element. Lets do an example:    

script >

script >

form  name ="login"  onsubmit ="return validate()" >
input  type ="text"  size ="20"  name ="userName" >
input  type ="text"  size ="20"  name ="password" >
input  type ="submit"  name ="submit"  value ="Submit" >
form >

Note:
if(document.login.userName.value=="").This means "If the box named userName of the form named login contains nothing, then...". return false. This is used to stop the form from submitting. By default, a form will return true if submitting. return validate() That means, "if submitting, then call the function validate() ". 

Protect a file by using Login

Let's try an example 

html > head >
SCRIPT  Language ="JavaScript" >
function  checkLogin(x)
{
if  ((x.id.value  !=   " Sam " ) || (x.pass.value  != " Sam123 " ))
{
    alert( " Invalid Login " );
     return   false ;
}
else
    location = " main.htm "
}
script >

head > body >
form >
p > UserID: input  type ="text"  name ="id" > p >
p > Password: input  type ="password"  name ="pass" > p >
p > input  type ="button"  value