JavaScripte最经典和权威的教程（基础入门）
源自http://www.w3schools.com/js/default.asp
Introduction to JavaScript
JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.
JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Mozilla, Firefox, Netscape, and Opera.
What You Should Already Know
Before you continue you should have a basic understanding of the following:
- HTML / XHTML
If you want to study these subjects first, find the tutorials on our Home page.
What is JavaScript?
- JavaScript was designed to add interactivity to HTML pages
- JavaScript is a scripting language
- A scripting language is a lightweight programming language
- A JavaScript consists of lines of executable computer code
- A JavaScript is usually embedded directly into HTML pages
- JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
- Everyone can use JavaScript without purchasing a license
Are Java and JavaScript the Same?
NO!
Java and JavaScript are two completely different languages in both concept and design!
Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.
What can a JavaScript Do?
- JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
- JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
- JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
- JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
- JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
- JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
- JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer
JavaScript How To ...
The HTML <script> tag is used to insert a JavaScript into an HTML page.
Examples
Write text
How to write text on a page.
Write text with formatting
How to format the text on your page with HTML tags.
How to Put a JavaScript Into an HTML Page
<html> |
The code above will produce this output on an HTML page:
| Hello World! |
Example Explained
To insert a JavaScript into an HTML page, we use the <script> tag (also use the type attribute to define the scripting language).
So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and ends:
<html> |
The word document.write is a standard JavaScript command for writing output to a page.
By entering the document.write command between the <script type="text/javascript"> and </script> tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page:
<html> |
Note: If we had not entered the <script> tag, the browser would have treated the document.write("Hello World!") command as pure text, and just write the entire line on the page.
Ending Statements With a Semicolon?
With traditional programming languages, like C++ and Java, each code statement has to end with a semicolon.
Many programmers continue this habit when writing JavaScript, but in general, semicolons are optional! However, semicolons are required if you want to put more than one statement on a single line.
How to Handle Older Browsers
Browsers that do not support JavaScript will display the script as page content. To prevent them from doing this, we may use the HTML comment tag:
<script type="text/javascript"> |
The two forward slashes at the end of comment line (//) are a JavaScript comment symbol. This prevents the JavaScript compiler from compiling the line.
JavaScript Where To ...
JavaScripts in the body section will be executed WHILE the page loads.
JavaScripts in the head section will be executed when CALLED.
Examples
Head section
Scripts that contain functions go in the head section of the document. Then we can be sure that the script is loaded before the function is called.
Body section
Execute a script that is placed in the body section.
External script
How to access an external script.
Where to Put the JavaScript
JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.
Scripts in the head section: Scripts to be executed when they are called, or when an event is triggered, go in the head section. When you place a script in the head section, you will ensure that the script is loaded before anyone uses it.
<html> |
Scripts in the body section: Scripts to be executed when the page loads go in the body section. When you place a script in the body section it generates the content of the page.
<html> |
Scripts in both the body and the head section: You can place an unlimited number of scripts in your document, so you can have scripts in both the body and the head section.
<html> |
Using an External JavaScript
Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.
To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.
Note: The external script cannot contain the <script> tag!
To use the external script, point to the .js file in the "src" attribute of the <script> tag:
<html> |
Note: Remember to place the script exactly where you normally would write the script!
JavaScript Variables
A variable is a "container" for information you want to store.
Examples
Variable
Variables are used to store data. This example will show you how.
Variables
A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value.
Rules for variable names:
- Variable names are case sensitive
- They must begin with a letter or the underscore character
IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!
Declare a Variable
You can create a variable with the var statement:
var strname = some value |
You can also create a variable without the var statement:
strname = some value |
Assign a Value to a Variable
You can assign a value to a variable like this:
var strname = "Hege" |
Or like this:
strname = "Hege" |
The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege".
Lifetime of Variables
When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.
If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.
JavaScript If...Else Statements
Conditional statements in JavaScript are used to perform different actions based on different conditions.
Examples
If statement
How to write an if statement.
If...else statement
How to write an if...else statement.
If..else if...else statement
How to write an if..else if...else statement.
Random link
This example demonstrates a link, when you click on the link it will take you to W3Schools.com OR to RefsnesData.no. There is a 50% chance for each of them.
Conditional Statements
Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.
In JavaScript we have the following conditional statements:
- if statement - use this statement if you want to execute some code only if a specified condition is true
- if...else statement - use this statement if you want to execute some code if the condition is true and another code if the condition is false
- if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed
- switch statement - use this statement if you want to select one of many blocks of code to be executed
If Statement
You should use the if statement if you want to execute some code only if a specified condition is true.
Syntax
if (condition) |
Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!
Example 1
<script type="text/javascript"> var d=new Date() |
Example 2
<script type="text/javascript"> var d=new Date() |
Note: When comparing variables you must always use two equals signs next to each other (==)!
Notice that there is no ..else.. in this syntax. You just tell the code to execute some code only if the specified condition is true.
If...else Statement
If you want to execute some code if a condition is true and another code if the condition is not true, use the if....else statement.
Syntax
if (condition) |
Example
<script type="text/javascript"> var d = new Date() |
If...else if...else Statement
You should use the if....else if...else statement if you want to select one of many sets of lines to execute.
Syntax
if (condition1) |
Example
<script type="text/javascript">
var d = new Date()
var time = d.getHours()
if (time<10)
{
document.write("<b>Good morning</b>")
}
else if (time>10 && time<16)
{
document.write("<b>Good day</b>")
}
else
{
document.write("<b>Hello World!</b>")
}
</script>
JavaScript Switch Statement
Conditional statements in JavaScript are used to perform different actions based on different conditions.
Examples
Switch statement
How to write a switch statement.
The JavaScript Switch Statement
You should use the switch statement if you want to select one of many blocks of code to be executed.
Syntax
switch(n) |
This is how it works: First we have a single expression n (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.
Example
<script type="text/javascript">
//You will receive a different greeting based
//on what day it is. Note that Sunday=0,
//Monday=1, Tuesday=2, etc.
var d=new Date()
theDay=d.getDay()
switch (theDay)
{
case 5:
document.write("Finally Friday")
break
case 6:
document.write("Super Saturday")
break
case 0:
document.write("Sleepy Sunday")
break
default:
document.write("I'm looking forward to this weekend!")
}
</script>
JavaScript Operators
Arithmetic Operators
| + | Addition | x=2 y=2 x+y |
4 |
| - | Subtraction | x=5 y=2 x-y |
3 |
| * | Multiplication | x=5 y=4 x*y |
20 |
| / | Division | 15/5 5/2 |
3 2.5 |
| % | Modulus (division remainder) | 5%2 10%8 10%2 |
1 2 0 |
| ++ | Increment | x=5 x++ |
x=6 |
| -- | Decrement | x=5 x-- |
x=4 |
Assignment Operators
| = | x=y | x=y |
| += | x+=y | x=x+y |
| -= | x-=y | x=x-y |
| *= | x*=y | x=x*y |
| /= | x/=y | x=x/y |
| %= | x%=y | x=x%y |
Comparison Operators
| == | is equal to | 5==8 returns false |
| === | is equal to (checks for both value and type) | x=5 y="5" x==y returns true |
| != | is not equal | 5!=8 returns true |
| > | is greater than | 5>8 returns false |
| < | is less than | 5<8 returns true |
| >= | is greater than or equal to | 5>=8 returns false |
| <= | is less than or equal to | 5<=8 returns true |
Logical Operators
| && | and | x=6 y=3 (x < 10 && y > 1) returns true |
| || | or | x=6 y=3 (x==5 || y==5) returns false |
| ! | not | x=6 y=3 !(x==y) returns true |
String Operator
A string is most often text, for example "Hello World!". To stick two or more string variables together, use the + operator.
txt1="What a very" |
The variable txt3 now contains "What a verynice day!".
To add a space between two string variables, insert a space into the expression, OR in one of the strings.
txt1="What a very" or txt1="What a very " |
The variable txt3 now contains "What a very nice day!".
Conditional Operator
JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.
Syntax
variablename=(condition)?value1:value2 |
Example
greeting=(visitor=="PRES")?"Dear President ":"Dear " |
If the variable visitor is equal to PRES, then put the string "Dear President " in the variable named greeting. If the variable visitor is not equal to PRES, then put the string "Dear " into the variable named greeting.
JavaScript Popup Boxes
In JavaScript we can create three kinds of popup boxes: Alert box, Confirm box, and Prompt box.
Examples
Alert Box
An alert box is often used if you want to make sure information comes through to the user.
When an alert box pops up, the user will have to click "OK" to proceed.
Syntax:
alert("sometext") |
Confirm Box
A confirm box is often used if you want the user to verify or accept something.
When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed.
If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false.
Syntax:
confirm("sometext") |
Prompt Box
A prompt box is often used if you want the user to input a value before entering a page.
When a prompt box pops up, the user will have to click either "OK" or "Cancel" to proceed after entering an input value.
If the user clicks "OK" the box returns the input value. If the user clicks "Cancel" the box returns null.
Syntax:
prompt("sometext","defaultvalue")
JavaScript Functions
A function is a reusable code-block that will be executed by an event, or when the function is called.
Examples
Function
How to call a function.
Function with arguments
How to pass a variable to a function, and use the variable in the function.
Function with arguments 2
How to pass variables to a function, and use these variables in the function.
Function that returns a value
How to let the function return a value.
A function with arguments, that returns a value
How to let the function find the product of two arguments and return the result.
JavaScript Functions
To keep the browser from executing a script when the page loads, you can put your script into a function.
A function contains code that will be executed by an event or by a call to that function.
You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file).
Functions can be defined both in the <head> and in the <body> section of a document. However, to assure that the function is read/loaded by the browser before it is called, it could be wise to put it in the <head> section.
Example
<html> <body> |
If the line: alert("Hello world!!") in the example above had not been put within a function, it would have been executed as soon as the line was loaded. Now, the script is not executed before the user hits the button. We have added an onClick event to the button that will execute the function displaymessage() when the button is clicked.
You will learn more about JavaScript events in the JS Events chapter.
How to Define a Function
The syntax for creating a function is:
function functionname(var1,var2,...,varX) |
var1, var2, etc are variables or values passed into the function. The { and the } defines the start and end of the function.
Note: A function with no parameters must include the parentheses () after the function name:
function functionname() |
Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.
The return Statement
The return statement is used to specify the value that is returned from the function.
So, functions that are going to return a value must use the return statement.
Example
The function below should return the product of two numbers (a and b):
function prod(a,b) |
When you call the function above, you must pass along two parameters:
product=prod(2,3) |
The returned value from the prod() function is 6, and it will be stored in the variable called product.
JavaScript For Loop
Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true.
Examples
For loop
How to write a for loop. Use a For loop to run the same block of code a specified number of times.
Looping through HTML headers
How to use the for loop to loop through the different HTML headers.
JavaScript Loops
Very often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
In JavaScript there are two different kind of loops:
- for - loops through a block of code a specified number of times
- while - loops through a block of code while a specified condition is true
The for Loop
The for loop is used when you know in advance how many times the script should run.
Syntax
for (var=startvalue;var<=endvalue;var=var+increment) |
Example
Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.
Note: The increment parameter could also be negative, and the <= could be any comparing statement.
<html> |
Result
The number is 0 |
JavaScript While Loop
Loops in JavaScript are used to execute the same block of code a specified number of times or while a specified condition is true.
Examples
While loop
How to write a while loop. Use a while loop to run the same block of code while a specified condition is true.
Do while loop
How to write a do...while loop. Use a do...while loop to run the same block of code while a specified condition is true. This loop will always be executed at least once, even if the condition is false, because the statements are executed before the condition is tested.
The while loop
The while loop is used when you want the loop to execute and continue executing while the specified condition is true.
while (var<=endvalue) |
Note: The <= could be any comparing statement.
Example
Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.
<html> |
Result
The number is 0 |
The do...while Loop
The do...while loop is a variant of the while loop. This loop will always execute a block of code ONCE, and then it will repeat the loop as long as the specified condition is true. This loop will always be executed at least once, even if the condition is false, because the code is executed before the condition is tested.
do |
Example
<html> |
Result
The number is 0 |
JavaScript Break and Continue
There are two special statements that can be used inside loops: break and continue.
Examples
Break statement
Use the break statement to break the loop.
Continue statement
Use the continue statement to break the current loop and continue with the next value.
JavaScript break and continue Statements
There are two special statements that can be used inside loops: break and continue.
Break
The break command will break the loop and continue executing the code that follows after the loop (if any).
Example
<html> |
Result
The number is 0 |
Continue
The continue command will break the current loop and continue with the next value.
Example
<html> |
Result
The number is 0 |
JavaScript For...In Statement
The for...in statement is used to loop (iterate) through the elements of an array or through the properties of an object.
Examples
For...In statement
How to use a for...in statement to loop through the elements of an array.
JavaScript For...In Statement
The for...in statement is used to loop (iterate) through the elements of an array or through the properties of an object.
The code in the body of the for ... in loop is executed once for each element/property.
Syntax
for (variable in object) |
The variable argument can be a named variable, an array element, or a property of an object.
Example
Using for...in to loop through an array:
<html> <script type="text/javascript"> </body> |
JavaScript Events
Events are actions that can be detected by JavaScript.
Events
By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript.
Every element on a web page has certain events which can trigger JavaScript functions. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags.
Examples of events:
- A mouse click
- A web page or an image loading
- Mousing over a hot spot on the web page
- Selecting an input box in an HTML form
- Submitting an HTML form
- A keystroke
Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!
For a complete reference of the events recognized by JavaScript, go to our complete Event reference.
onload and onUnload
The onload and onUnload events are triggered when the user enters or leaves the page.
The onload event is often used to check the visitor's browser type and browser version, and load the proper version of the web page based on the information.
Both the onload and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user's name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: "Welcome John Doe!".
onFocus, onBlur and onChange
The onFocus, onBlur and onChange events are often used in combination with validation of form fields.
Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field:
<input type="text" size="30" |
onSubmit
The onSubmit event is used to validate ALL form fields before submitting it.
Below is an example of how to use the onSubmit event. The checkForm() function will be called when the user clicks the submit button in the form. If the field values are not accepted, the submit should be cancelled. The function checkForm() returns either true or false. If it returns true the form will be submitted, otherwise the submit will be cancelled:
<form method="post" action="xxx.htm" |
onMouseOver and onMouseOut
onMouseOver and onMouseOut are often used to create "animated" buttons.
Below is an example of an onMouseOver event. An alert box appears when an onMouseOver event is detected:
<a href="http://www.w3schools.com" |
JavaScript Try...Catch Statement
The try...catch statement allows you to test a block of code for errors.
Examples
The try...catch statement
How to write a try...catch statement.
The try...catch statement with a confirm box
Another example of how to write a try...catch statement.
JavaScript - Catching Errors
When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page.
This chapter will teach you how to trap and handle JavaScript error messages, so you don't lose your audience.
There are two ways of catching errors in a Web page:
- By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6)
- By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3)
Try...Catch Statement
The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.
Syntax
try |
Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error!
Example 1
The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:
<html> |
To take more appropriate action when an error occurs, you can add a try...catch statement.
The example below contains the "Welcome guest!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, the catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:
<html> |
Example 2
The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:
<html> <body> </html> |
The onerror Event
The onerror event will be explained soon, but first you will learn how to use the throw statement to create an exception. The throw statement can be used together with the try...catch statement.
JavaScript Throw Statement
The throw statement allows you to create an exception.
Examples
The throw statement
How to use the throw statement.
The Throw Statement
The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.
Syntax
throw(exception) |
The exception can be a string, integer, Boolean or an object.
Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!
Example 1
The example below determines the value of a variable called x. If the value of x is higher than 10 or lower than 0 we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:
<html>
<body>
<script type="text/javascript">
var x=prompt("Enter a number between 0 and 10:","")
try
{
if(x>10)
throw "Err1"
else if(x<0)
throw "Err2"
}
catch(er)
{
if(er=="Err1")
alert("Error! The value is too high")
if(er == "Err2")
alert("Error! The value is too low")
}
</script>
</body>
</html>
JavaScript The onerror Event
Using the onerror event is the old standard solution to catch errors in a web page.
Examples
The onerror event (an example with an error)
How to use the onerror event to catch errors in a web page.
The onerror Event
We have just explained how to use the try...catch statement to catch errors in a web page. Now we are going to explain how to use the onerror event for the same purpose.
The onerror event is fired whenever there is a script error in the page.
To use the onerror event, you must create a function to handle the errors. Then you call the function with the onerror event handler. The event handler is called with three arguments: msg (error message), url (the url of the page that caused the error) and line (the line where the error occurred).
Syntax
onerror=handleErr function handleErr(msg,url,l) |
The value returned by onerror determines whether the browser displays a standard error message. If you return false, the browser displays the standard error message in the JavaScript console. If you return true, the browser does not display the standard error message.
Example
The following example shows how to catch the error with the onerror event:
<html> function handleErr(msg,url,l) function message() <body> </html> |
JavaScript Special Characters
In JavaScript you can add special characters to a text string by using the backslash sign.
Insert Special Characters
The backslash (\) is used to insert apostrophes, new lines, quotes, and other special characters into a text string.
Look at the following JavaScript code:
var txt="We are the so-called "Vikings" from the north." |
In JavaScript, a string is started and stopped with either single or double quotes. This means that the string above will be chopped to: We are the so-called
To solve this problem, you must place a backslash (\) before each double quote in "Viking". This turns each double quote into a string literal:
var txt="We are the so-called \"Vikings\" from the north." |
JavasScript will now output the proper text string: We are the so-called "Vikings" from the north.
Here is another example:
document.write ("You \& me are singing!") |
The example above will produce the following output:
You & me are singing! |
The table below lists other special characters that can be added to a text string with the backslash sign:
| \' | single quote |
| \" | double quote |
| \& | ampersand |
| \\ | backslash |
| \n | new line |
| \r | carriage return |
| \t | tab |
| \b | backspace |
| \f | form feed |
JavaScript Guidelines
Some other important things to know when scripting with JavaScript.
JavaScript is Case Sensitive
A function named "myfunction" is not the same as "myFunction" and a variable named "myVar" is not the same as "myvar".
JavaScript is case sensitive - therefore watch your capitalization closely when you create or call variables, objects and functions.
White Space
JavaScript ignores extra spaces. You can add white space to your script to make it more readable. The following lines are equivalent:
name="Hege" |
Break up a Code Line
You can break up a code line within a text string with a backslash. The example below will be displayed properly:
document.write("Hello \ |
However, you cannot break up a code line like this:
document.write \ |
Comments
You can add comments to your script by using two slashes //:
//this is a comment |
or by using /* and */ (this creates a multi-line comment block):
/* This is a comment
block. It contains
several lines */
document.write("Hello World!")