JavaScript Tutorial
JavaScript Tutorial ——指南
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| JavaScript is the scripting language of the Web! JavaScript是一种工作在Web页面的脚本语言！ JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more. JavaScript已经在无数Web页面发挥着作用，他们提升设计效果、验证表单、检测浏览器、创建cookies，等等等等。 JavaScript is the most popular scripting language on the internet. JavaScript是Internet上最流行的脚本语言。 JavaScript is easy to learn! You will enjoy it! JavaScript容易上手！您可以轻松享受！ |
JavaScript Examples ——举例
Learn by 100 examples! With our editor, you can edit the source code, and click on a test button to view the result.
JavaScript学习100例！通过你的编辑器，您可以编辑源代码，并通过点击测试键（test）看到结果。
JavaScript Quiz Test ——测试
Test your JavaScript skills at W3Schools!
在W3Schools测试您的JavaScript技术！
Start JavaScript Quiz! 开始测试JavaScript
JavaScript References ——JavaScript说明（rfc?）
At W3Schools you will find complete references of all JavaScript objects and the HTML DOM objects.
您可以在W3Schools找到全部JavaScript的对象和HTML Dom 对象的完整文档。
Complete reference of all JavaScript objects. With examples! ——带例子的全部JavaScript对象完整文档
Complete reference of all HTML DOM objects. With examples! ——带例子的全部HTML DOM对象完整文档
JavaScript介绍
JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.
JavaScript已经在无数Web页面发挥着作用，他们提升设计效果、验证表单、检测浏览器、创建cookies，等等等等
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.
JavaScript是Internet上最流行的脚本语言，并且在主流浏览器上都能运行，比如IE，Mozilla，Firefox,Netscape，Opera。
What You Should Already Know ——需要首先掌握的知识
Before you continue you should have a basic understanding of the following:
在您开始学习之前，首先应该对下列知识有一些了解
HTML / XHTML
If you want to study these subjects first, find the tutorials on our Home page.
如果您打算先学习这些课题，可以在我们的主页上找到相关指南
What is JavaScript?JavaScript是什么？
JavaScript was designed to add interactivity to HTML pages
JavaScript是为了增强HTML页面的交互性而出现的
JavaScript is a scripting language
JavaScript是一种脚本语言
A scripting language is a lightweight programming language
脚本语言是一种轻量级的程序语言
A JavaScript consists of lines of executable computer code
JavaScript最终成为可执行的机器代码中的一部分
A JavaScript is usually embedded directly into HTML pages
JavaScript通常直接嵌入到HTML页面中
JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
JavaScript是一种解释性语言（这意味着脚本的执行将不需要进行另外的编译）
Everyone can use JavaScript without purchasing a license
任何人都可以使用JavaScript而不需要购买什么许可^_^
Are Java and JavaScript the Same? Java和JavaScript一样么？
NO!
不一样！
Java and JavaScript are two completely different languages in both concept and design!
无论是从概念还是从设计上看，Java与JavaScript都是两种完全不同的语言
Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.
Java（Sun公司设计出来的东西）是一种强大的并且十分复杂的程序语言——与C、C++属于同一家族
What can a JavaScript Do? JavaScript能做什么？
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为HTML设计人员提供了一种编程工具——HTML作者通常并不是程序员，而JavaScript这种脚本语言使用的语法非常简单！啥人都能在HTML页面中写这么个小玩意儿出来
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可以把动态文本插入到HTML页面中——如果想写入一段动态文本，JavaScript段就是是介个样子滴：document.write("<h1>" + name + "</h1>")
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
可以通过时间重新激活JavaScript——可以设定JavaScript在某件事情发生时随之执行，比如页面载入完成啦，或者用户点击了HTML上的什么东东啦
JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
JavaScript可以对HTML元素进行读写——JavaScript可以读取甚至改变HTML元素的内容
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可以拿来验证数据——在表单提交到数据库之前，我们可以用JavaScript来验证表单数据。这样做可以降低服务器的负荷
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可以检测访问者的浏览器——JavaScript可以检测访问者的浏览器，并且加载一个为这个浏览器设计的页面
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer
JavaScript可以创建cookies——JavaScript可以存储和查询存放在访问者的电脑中的（cookies）信息
JavaScript 应该怎么写
The HTML <script> tag is used to insert a JavaScript into an HTML page.
HTML中使用<script>标签将JavaScript插入到HTML页面中。
Examples 例子
Write text 写一段文本
How to write text on a page. 如何在页面上写一段文字。
Write text with formatting 写带格式的文本
How to format the text on your page with HTML tags. 如何在你的页面上使用HTML标签写一段带格式的文本内容。
How to Put a JavaScript Into an HTML Page 如何把一段JavaScript代码加入到HTML页面中
<html> |
The code above will produce this output on an HTML page: 上面这段代码将在HTML页面上产生这样一段输出效果：
| Hello World! |
Example Explained 例子解释
To insert a JavaScript into an HTML page, we use the <script> tag (also use the type attribute to define the scripting language).
我们可以通过<script> 标签将JavaScript插入到HTML页面中。（同时还要用 type 属性定义使用的脚本语言）
So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and ends:
像这样用<script type="text/javascript">和</script>声明JavaScript的起止：
<html> |
The word document.write is a standard JavaScript command for writing output to a page.
document.write 这段字是JavaScript向页面输出内容的标准命令。
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:
通过在 <script type="text/javascript"> 和 </script> 标签之间输入 document.write 命令，浏览器会自动识别这段JavaScript命令并执行。下面演示了通过这种方法在页面中输出Hello world！
<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.
注意：如果我们没有输入<script>标签，浏览器会将 document.write("Hello World!") 这段命令当作一段纯文本内容，并且直接把这条命令整行输出到页面上。
Ending Statements With a Semicolon? 用分号作语句结尾？
With traditional programming languages, like C++ and Java, each code statement has to end with a semicolon.
像C++和Java这类复杂的程序语言中，通常是用分号作为一个表达式或者语句的结尾
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.
在写JavaScript的时候很多程序员希望保留这种习惯，尽管分号并不是必需的，然而，当您输入的是不止一个语句的时候，那么使用分号仍然是必要的。
How to Handle Older Browsers 老版本浏览器怎么办
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:
不支持JavaScript的浏览器会把脚本作为页面内容输出的。要防止这种情况，我们可以使用下面这个HTML标签：
<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.
命令行前面的两个正斜杠是JavaScript的注释标记。这将防止JavaScript编译器编译这一行。
JavaScript 应该写在哪
JavaScripts in the body section will be executed WHILE the page loads. 写在body段里的JavaScript会在页面读取过程中执行。
JavaScripts in the head section will be executed when CALLED. 写在head段里的JavaScript会在被调用的时候执行
Examples 例子
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.
包含函数的脚本写在文档的head段。在函数被调用之前，脚本就已经被加载好了，可以随时调用
Body section
Execute a script that is placed in the body section.
执行放在body段里的脚本。
External script
How to access an external script.
如何访问外部脚本。
Where to Put the JavaScript 该把JavaScript放在哪
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.
可能有时候我们会对JavaScript的执行时间有些自己的想法：有时候我们不希望页面上的JavaScript在浏览器加载的时候立刻执行，但也可能有时候我们希望在页面加载的时候就执行一些脚本。
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.
写在和head段的脚本：head段里的脚本在被调用的时候执行，或者在事件被触发的时候执行。不过放在head段的脚本，却是在被调用前就加载好的。
<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.
body段的脚本：body段里的脚本在页面加载的时候就立刻执行了。当你把脚本放在body段里的时候，它会生成一些页面的内容。
<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.
body和head段都可以有脚本：你可以在你的文档中安排若干个脚本，所以在head和body段里可以同时各自写有脚本。
<html> |
Using an External JavaScript 使用外部JavaScript
Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.
有时您可能想要在不同的页面执行同一个Javascript，而又不想在每一个页面分别写一次同样的脚本。
To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.
为了简化这种工作，您可以使用外部JavaScript。把外部JavaScript文件保存到一个以.js为扩展名的文件中。
Note: The external script cannot contain the <script> tag!
注意：在外部脚本里不能包含<script>标签！
To use the external script, point to the .js file in the "src" attribute of the <script> tag:
使用外部脚本时，需要在<script>标签中将.js文件在“src”属性中指明：
<html> |
Note: Remember to place the script exactly where you normally would write the script!
注意：记得把脚本放在你平时写脚本的地方
JavaScript 中的变量
A variable is a "container" for information you want to store.
变量是您用来存储信息的容器。
Examples 例子
Variable 变量
Variables are used to store data. This example will show you how.
变量是用来存储数据的。这个例子向您展示如何使用。
Variables 变量
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.
变量是您用来存储信息的一个容器。在脚本执行过程中，变量的值是可以改变的。无论值怎么变，您都可以通过变量名来使用这个变量。
Rules for variable names: 变量命名规则：
Variable names are case sensitive 变量名是大小写敏感的
They must begin with a letter or the underscore character 必须以字母或下划线开头
IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!
重要信息！JavaScript是大小写敏感的！比如strname和STRNAME这两个变量是不一样的！
Declare a Variable 声明变量
You can create a variable with the var statement: 您可以通过下述var语句建立变量：
var strname = some value |
You can also create a variable without the var statement: 您也可以通过这种没有var的语句建立变量
strname = some value |
Assign a Value to a Variable 给变量赋值
You can assign a value to a variable like this: 可以这样赋值：
var strname = "Hege" |
Or like this: 这样也行：
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".
变量名必须写在赋值表达式左边。
Lifetime of Variables 变量的生命周期
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.
当您在一个函数中声明了一个变量，那么这个变量只能在函数内部使用，函数结束的时候这个变量就被销毁。这叫做局部变量。您也可以在不同函数中用相同的变量名再声明一个变量，因为只有在函数内部才能识别这个变量。
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.
如果您在函数外部声明了一个变量，那么所有的函数都能访问这个变量。这个变量的生命周期从声明时开始，直到页面关闭时才结束。
JavaScript If...Else 条件表达式
Conditional statements in JavaScript are used to perform different actions based on different conditions.
JavaScript的条件表达式用来对不同条件作出不同响应。
Examples 例子
If statement If 表达式
How to write an if statement. 如何写if表达式
If...else statement If...else表达式
How to write an if...else statement. 如何写if...else表达式。
If..else if...else statement If..else if...else表达式
How to write an if..else if...else statement. 如何写 if..else if...else 表达式
Random link 随即链接
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.
这个例子演示了一个链接，当您点击这个链接的时候，它将带您到 W3Schools.com 或者 RefsnesData.no。各有50%机率。
Conditional Statements 条件表达式
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.
在您写代码的时候，经常需要面对不同选择做出不同响应。这时您可以使用判断语句。
In JavaScript we have the following conditional statements:
在JavaScript中，我们可以使用下列判断语句
if statement - use this statement if you want to execute some code only if a specified condition is true
if 语句 - 如果您只是想制定当某一状态为true时需要执行操作，则使用这种语句
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 语句 - 如果您想分别针对状态为真或假时指定动作，则使用这种语句
if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed
if...else if....else 语句 - 如果你想对多种情况择一运行，则使用这种语句
switch statement - use this statement if you want to select one of many blocks of code to be executed
switch 语句 - 效果与前一种类似，但语法不同
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"> |
Example 2
<script type="text/javascript"> |
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"> |
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"> |
JavaScript的Switch 语句
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"> |
JavaScript 操作符
Arithmetic Operators
| Operator | Description | Example | Result |
|---|---|---|---|
| + | Addition 加法 | x=2 | 4 |
| - | Subtraction 减法 | x=5 | 3 |
| * | Multiplication 乘法 | x=5 | 20 |
| / | Division 除法 | 15/5 | 3 |
| % | Modulus (division remainder) 取模(余) | 5%2 | 1 |
| ++ | Increment 自增 | x=5 | x=6 |
| -- | Decrement 自减 | x=5 | x=4 |
Assignment Operators
| Operator | Example | Is The Same As |
|---|---|---|
| = | 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 比较运算符
| Operator | Description | Example |
|---|---|---|
| == | is equal to 相等 | 5==8 returns false |
| === | is equal to (checks for both value and type) | x=5 x==y returns true |
| != | is not equal 不等 | 5!=8 returns true |
| > | is greater than 大于 | 5>8 returns false |
| < | is less than 小于 | 5<8 returns true |
| >= | is greater than or equal to 大于等于 | 5>=8 returns false |
| <= | is less than or equal to 小于等于 | 5<=8 returns true |
Logical Operators
| Operator | Description | Example |
|---|---|---|
| && | and 与 | x=6 (x < 10 && y > 1) returns true |
| || | or 或 | x=6 (x==5 || y==5) returns false |
| ! | not 非 | x=6 !(x==y) returns true |
String Operator 字符串操作符
A string is most often text, for example "Hello World!". To stick two or more string variables together, use the + operator.
字符串时最常用的文字类型，例如：“Hello World！”。可以用两个字符串直接使用“+”号相加
txt1="What a very" |
The variable txt3 now contains "What a verynice day!".
这时，变量txt3中的内容是“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" |
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弹出对话框
In JavaScript we can create three kinds of popup boxes: Alert box, Confirm box, and Prompt box.
在JavaScript中，我们可以创建三中弹出式对话框：警告，提醒，和对话框
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.
如果您想判断用户对某事是否进行确认或接受，可以使用确认框
When a confirm box pops up, the user will have to click either "OK" or "Cancel" to proceed.
弹出确认框时，将对用户点击OK或Cancel进行处理
If the user clicks "OK", the box returns true. If the user clicks "Cancel", the box returns false.
如果用户点击OK，返回值为true。如果用户点击Cancel，返回值为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.
用户点击OK是返回输入的值。用户点击Cancel时返回值为空
Syntax:
prompt("sometext","defaultvalue") |
JavaScript函数
A function is a reusable code-block that will be executed by an event, or when the function is called.
函数是一个代码块，在被函数或事件调用时被整体执行。
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 2 arguments and return the result.
JavaScript 函数
To keep the browser from executing a script as soon as the page is loaded, you can write your script as a function.
浏览器载入页面时，不会立刻执行写在函数中的代码。
A function contains some code that will be executed only by an event or by a call to that function.
函数中的代码只有在函数被调用的时候才会执行，比如触发事件之类的。
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).
任何东东都可以调用函数，但不能超出页面范围（有一种例外：函数被写在“.js”后缀的文件中时，文件可以被其他页面调用）
Functions are defined at the beginning of a page, in the <head> section.
函数定义应该写在页面的开头，也就是<head>段中。
Example
<html> |
If the line: alert("Hello world!!"), in the example above had not been written 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.
如何定义一个函数
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.
返回语句
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的For循环
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 |
The while loop
The while loop will be explained in the next chapter.
JavaScript的While循环
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的中止和继续
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的For...In语句
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> |
JavaScript事件
Events are actions that can be detected by JavaScript.
事件，是可以被JavaScript检测到的动作：
Events
By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript.
通过使用JavaScript，我们可以创建web页面。事件是可以被JavaScript检测到的动作。
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.
每一个页面元素都有一些特定的事件可以激活JavaScript函数。例如，我们可以使用按钮元素的onClick事件响应用户的点击动作。我们在HTML标签中定义这些事件。
Examples of events:
事件举例：
A mouse click 鼠标点击
A web page or an image loading 页面或图像载入
Mousing over a hot spot on the web page 鼠标划过页面的某些hot spot?
Selecting an input box in an HTML form 在HTML表单中选择了一个输入框
Submitting an HTML form 提交HTML表单
A keystroke @#￥%某个什么东西……
Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!
注意：事件通常用来激活某个函数，而这个函数在事件发生前并不运行。
For a complete reference of the events recognized by JavaScript, go to our complete Event reference.
在我们的完整事件说明中，有JavaScript支持的全部事件的介绍
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的Try...Catch语句
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> |
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的抛出语句
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> |
JavaScript的onerror事件
在web页面中捕获错误的一个古老而通用的方法，就是使用onerror事件。
Examples
The onerror event (an example with an error)
How to use the onerror event to catch errors in a web page.
关于 onerror 事件
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.
我们刚刚介绍过如何通过try...catch语句捕获页面中的错误。现在我们再来看看如何通过onerror时间来达到同样的目的。
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).
使用onerror事件前，我们需要一个用来处理错误的函数。当然你可以管这个函数叫onerror事件处理器。这个事件处理器调用三个参数：msg(错误消息),url(导致错误发生的页面url)和line(错误发生的代码所在行)
Syntax
onerror=handleErr function handleErr(msg,url,l) { //Handle the error here return true or false } |
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> |
JavaScript中的特殊字符
在JavaScript中如果需要添加特殊字符，就需要在字符前加上反斜线。
Insert Special Characters
插入特殊字符
如果需要在字符串中增加换行、引号等特殊字符，需要在字符前增加反斜线（）
请看下面的JavaScript代码：
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:
| Code | Outputs |
|---|---|
| ' | single quote |
| " | double quote |
| & | ampersand |
| \ | backslash |
| n | new line |
| r | carriage return |
| t | tab |
| b | backspace |
| f | form feed |
JavaScript手册
Some other important things to know when scripting with JavaScript.
写JavaScript脚本时，还要知道些重要的东西
JavaScript是大小写敏感的
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.
关于空格
JavaScript ignores extra spaces. You can add white space to your script to make it more readable. The following lines are equivalent:
name="Hege" |
关于代码中的换行
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 |
关于注释
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 |
