Do I need to declare variables when using ecmascript?
ecmascript使用变量不需要声明。在ecmascript中,使用变量之前不必声明,变量声明不是必须的;原因:ECMAScript的解释程序遇到未声明过的标识符时,会用该变量名创建一个全局变量,并将其初始化为指定的值。
本教程操作环境:windows7系统、ECMAScript 6版、Dell G3电脑。
ECMAScript 变量无需明确的类型声明
变量名需要遵守一些简单的规则。
声明变量
ECMAScript 中的变量是用 var 运算符(variable 的缩写)加变量名定义的。例如:
var test = "hi";
在这个例子中,声明了变量 test,并把它的值初始化为 "hi"(字符串)。由于 ECMAScript 是弱类型的,所以解释程序会为 test 自动创建一个字符串值,无需明确的类型声明。
还可以用一个 var 语句定义两个或多个变量:
var test1 = "hi", test2 = "hello";
前面的代码定义了变量 test1,初始值为 "hi",还定义了变量 test2,初始值为 "hello"。
不过用同一个 var 语句定义的变量不必具有相同的类型,如下所示:
var test = "hi", age = 25;
这个例子除了(再次)定义 test 外,还定义了 age,并把它初始化为 25。即使 test 和 age 属于两种不同的数据类型,在 ECMAScript 中这样定义也是完全合法的。
与 Java 不同,ECMAScript 中的变量并不一定要初始化(它们是在幕后初始化的,将在后面讨论这一点)。因此,下面这一行代码也是有效的:
var test;
此外,与 Java 不同的还有变量可以存放不同类型的值。这是弱类型变量的优势。例如,可以把变量初始化为字符串类型的值,之后把它设置为数字值,如下所示:
var test = "hi"; alert(test); test = 55; alert(test);
这段代码将毫无问题地输出字符串值和数字值。但是,如前所述,使用变量时,好的编码习惯是始终存放相同类型的值。
命名变量
变量名需要遵守两条简单的规则:
- 第一个字符必须是字母、下划线(_)或美元符号($)
- 余下的字符可以是下划线、美元符号或任何字母或数字字符
下面的变量都是合法的:
var test; var $test; var $1; var _$te$t2;
著名的变量命名规则
只是因为变量名的语法正确,并不意味着就该使用它们。变量还应遵守以下某条著名的命名规则:
Camel 标记法
首字母是小写的,接下来的字母都以大写字符开头。例如:
var myTestValue = 0, mySecondValue = "hi";
Pascal 标记法
首字母是大写的,接下来的字母都以大写字符开头。例如:
var MyTestValue = 0, MySecondValue = "hi";
匈牙利类型标记法
在以 Pascal 标记法命名的变量前附加一个小写字母(或小写字母序列),说明该变量的类型。例如,i 表示整数,s 表示字符串,如下所示“
var iMyTestValue = 0, sMySecondValue = "hi";
本教程采用了这些前缀,以使示例代码更易阅读:
类型 | 前缀 | 示例 |
---|---|---|
数组 | a | aValues |
布尔型 | b | bFound |
浮点型(数字) | f | fValue |
函数 | fn | fnMethod |
整型(数字) | i | iValue |
对象 | o | oType |
正则表达式 | re | rePattern |
字符串 | s | sValue |
变型(可以是任何类型) | v | vValue |
变量声明不是必须的
ECMAScript 另一个有趣的方面(也是与大多数程序设计语言的主要区别),是在使用变量之前不必声明。例如:
var sTest = "hello "; sTest2 = sTest + "world"; alert(sTest2);
在上面的代码中,首先,sTest 被声明为字符串类型的值 "hello"。接下来的一行,用变量 sTest2 把 sTest 与字符串 "world" 连在一起。变量 sTest2 并没有用 var 运算符定义,这里只是插入了它,就像已经声明过它一样。
ECMAScript 的解释程序遇到未声明过的标识符时,用该变量名创建一个全局变量,并将其初始化为指定的值。
这是该语言的便利之处,不过如果不能紧密跟踪变量,这样做也很危险。最好的习惯是像使用其他程序设计语言一样,总是声明所有变量。
【相关推荐:javascript学习教程】
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