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版本 276558 / 变量

  • 版本网址缩略名: Core_JavaScript_1.5_Guide/变量
  • 版本标题: 变量
  • 版本 id: 276558
  • 创建于:
  • 创建者: Cnmahj
  • 是否是当前版本?
  • 评论 /* Variables */ 变量的说明 完成

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变量

在应用程序中使用变量作为值的符号名。变量名又叫做“标识符”,要符合一定规则。

一个JavaScript标识符必须以字母、下划线(_)或者美元符号($)作为首字符。随后的字符也可以是数字。因为JavaScript大小写敏感,所以字母包含“A”到“Z”(大写)和“a”到“z”(小写)。

从JavaScript 1.5开始,你可以在标识符中使用ISO 8859-1或者Unicode字母,比如“å”和“ü”。你也可以在标识符中使用转义序列页面的\uXXXX Unicode转义序列。

合格的标识符如Number_hitstemp99_name

定义变量

你可以使用两种方式来定义变量:

  • 使用关键字 var。例如, var x = 42。 这个语法可以用来声明局部和全局变量。
  • 对变量进行简单赋值。 例如, x = 42。 这常常用来声明全局变量,并生成一个严格的JavaScript警告。 你不应该使用形式。

Evaluating Variables

A variable declared using the var statement with no initial value specified has the value undefined.

An attempt to access an undeclared variable will result in a ReferenceError exception being thrown:

var a;
print("The value of a is " + a); // prints "The value of a is undefined"
print("The value of b is " + b); // throws ReferenceError exception

You can use undefined to determine whether a variable has a value. In the following code, the variable input is not assigned a value, and the if statement evaluates to true.

var input;
if(input === undefined){
  doThis();
} else {
  doThat();
}

Not sure how the following is related to "Variables" section The undefined value behaves as false when used in a boolean context. For example, the following code executes the function myFunction because the myArray element is not defined:

var myArray = new Array();
if (!myArray[0]) myFunction();

When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in boolean contexts. For example:

var n = null;
print(n * 32); // prints 0

Variable Scope

When you declare a variable outside of any function, it is called a global variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a local variable, because it is available only within that function.

JavaScript does not have block statement scope; rather, it will be local to the code that the block resides within. For example the following code will print 5 instead of throwing if condition is false:

if (condition) {
  var x = 5;
}
print(x ? x : 0);

Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception:

print(x === undefined); // prints "true"
var x = 3;

Global Variables

need links to pages discussing scope chains and the global object Global variables are in fact properties of the global object. In web pages the global object is window, so you can set and access global variables using the window.variable syntax.

Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called phoneNumber is declared in a FRAMESET document, you can refer to this variable from a child frame as parent.phoneNumber.

See Also

Sharp variables in JavaScript

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修订版来源

<p>
</p>
<h3 name=".E5.8F.98.E9.87.8F"> 变量 </h3>
<p>在应用程序中使用变量作为值的符号名。变量名又叫做“标识符”,要符合一定规则。
</p><p>一个JavaScript标识符必须以字母、下划线(_)或者美元符号($)作为首字符。随后的字符也可以是数字。因为JavaScript大小写敏感,所以字母包含“A”到“Z”(大写)和“a”到“z”(小写)。
</p><p>从JavaScript 1.5开始,你可以在标识符中使用ISO 8859-1或者Unicode字母,比如“å”和“ü”。你也可以在标识符中使用<a href="cn/Core_JavaScript_1.5_Guide/Unicode#Unicode_Escape_Sequences">转义序列</a>页面的\uXXXX Unicode转义序列。
</p><p>合格的标识符如<code>Number_hits</code>、<code>temp99</code>和<code>_name</code>。
</p>
<h4 name=".E5.AE.9A.E4.B9.89.E5.8F.98.E9.87.8F"> 定义变量 </h4>
<p>你可以使用两种方式来定义变量:
</p>
<ul><li> 使用关键字 <a href="cn/Core_JavaScript_1.5_Reference/Statements/var">var</a>。例如, <code>var x = 42</code>。 这个语法可以用来声明<a href="#Variable_Scope">局部和全局</a>变量。
</li><li> 对变量进行简单赋值。 例如, <code>x = 42</code>。 这常常用来声明<a href="#Global_Variables">全局变量</a>,并生成一个严格的JavaScript警告。 你不应该使用形式。
</li></ul>
<h4 name="Evaluating_Variables"> Evaluating Variables </h4>
<p>A variable declared using the <code>var</code> statement with no initial value specified has the value <a href="cn/Core_JavaScript_1.5_Reference/Global_Properties/undefined">undefined</a>.
</p><p>An attempt to access an undeclared variable will result in a ReferenceError exception being thrown:
</p>
<pre class="eval">var a;
print("The value of a is " + a); // prints "The value of a is undefined"
print("The value of b is " + b); // throws ReferenceError exception
</pre>
<p>You can use <code>undefined</code> to determine whether a variable has a value. In the following code, the variable <code>input</code> is not assigned a value, and the <code><a href="cn/Core_JavaScript_1.5_Reference/Statements/if...else">if</a></code> statement evaluates to <code>true</code>.
</p>
<pre class="eval">var input;
if(input === undefined){
  doThis();
} else {
  doThat();
}
</pre>
<p><span class="comment">Not sure how the following is related to "Variables" section</span>
The <code>undefined</code> value behaves as <code>false</code> when used in a boolean context. For example, the following code executes the function <code>myFunction</code> because the <code>myArray</code> element is not defined:
</p>
<pre class="eval">var myArray = new Array();
if (!myArray[0]) myFunction();
</pre>
<p>When you evaluate a null variable, the null value behaves as 0 in numeric contexts and as false in boolean contexts. For example:
</p>
<pre class="eval">var n = null;
print(n * 32); // prints 0
</pre>
<h4 name="Variable_Scope"> Variable Scope </h4>
<p>When you declare a variable outside of any function, it is called a <i>global</i> variable, because it is available to any other code in the current document. When you declare a variable within a function, it is called a <i>local</i> variable, because it is available only within that function.
</p><p>JavaScript does not have <a href="cn/Core_JavaScript_1.5_Guide/Block_Statement#Block_Statement">block statement</a> scope; rather, it will be local to the code that the block resides within. For example the following code will print 5 instead of throwing if <code>condition</code> is <code>false</code>:
</p>
<pre class="eval">if (condition) {
  var x = 5;
}
print(x ? x : 0);
</pre>
<p>Another unusual thing about variables in JavaScript is that you can refer to a variable declared later, without getting an exception:
</p>
<pre class="eval">print(x === undefined); // prints "true"
var x = 3;
</pre>
<h4 name="Global_Variables"> Global Variables </h4>
<p><span class="comment">need links to pages discussing scope chains and the global object</span>
Global variables are in fact properties of the <i>global object</i>. In web pages the global object is <a href="cn/DOM/window">window</a>, so you can set and access global variables using the <code>window.<i>variable</i></code> syntax.
</p><p>Consequently, you can access global variables declared in one window or frame from another window or frame by specifying the window or frame name. For example, if a variable called <code>phoneNumber</code> is declared in a <code>FRAMESET</code> document, you can refer to this variable from a child frame as <code>parent.phoneNumber</code>.
</p>
<h4 name="See_Also"> See Also </h4>
<p><a href="cn/Sharp_variables_in_JavaScript">Sharp variables in JavaScript</a>
</p><p>{{template.PreviousNext("Core_JavaScript_1.5_Guide:Values", "Core_JavaScript_1.5_Guide:常量")}}
</p>
<div class="noinclude">
</div>
{{ wiki.languages( { "es": "es/Gu\u00eda_JavaScript_1.5/Variables", "fr": "fr/Guide_JavaScript_1.5/Variables", "ja": "ja/Core_JavaScript_1.5_Guide/Variables", "ko": "ko/Core_JavaScript_1.5_Guide/Variables", "pl": "pl/Przewodnik_po_j\u0119zyku_JavaScript_1.5/Zmienne", "zh-cn": "cn/Core_JavaScript_1.5_Guide/\u53d8\u91cf" } ) }}
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