首先对javascript的类型再熟悉一?/strong>
javascript中变?variable)的?value)有这样几U类?type)Q?br />(ECMAscript Language Specification Edition 3 24-Mar-00)
8.1 The Undefined Type
The
Undefined type has exactly one value, called undefined. Any variable
that has not been assigned a value has the value undefined.
8.2 The Null Type
The Null type has exactly one value, called null.
8.3 The Boolean Type
The Boolean type represents a logical entity having two values, called true and false.
8.4 The String Type
The String type is the set of all finite ordered sequences of zero or more 16-bit unsigned integer values
(“elements?.
8.5 The Number Type
The Number type has exactly 18437736874454810627 (that is, 264?53+3) values, representing the doubleprecision
64-bit format IEEE 754 values as specified in the IEEE Standard for Binary Floating-Point Arithmetic,
except that the 9007199254740990 (that is, 253?) distinct “Not-a-Number?values of the IEEE Standard are
represented in ECMAscript as a single special NaN value. (Note that the NaN value is produced by the program
expression NaN, assuming that the globally defined variable NaN has not been altered by program execution.)
8.6 The Object Type
An Object is an unordered collection of properties. Each property consists of a name, a value and a set of attributes.
我们比较x面向对象Q所以对其中的Objectcd多加留意?br />如上面的定义QObjectcd是无序的属性的集合。每个属?property)有名Uͼ值和一些性质(attributes)Q如只读Q可枚DQ不可删Q内部的Q?br />
丑և个例子：
var a;//a现在对应一个|该值的cd?span style="FONT-WEIGHT: bold">Undefined
a = 1;//现在a对应一个Numbercd的?br />a = true;//现在a的类型变了，变成一个Booleancd的倹{?br />a = "x";//a现在对应一个Stringcd的?br />//Q注?x"和new String("x")q不相同Q?br />//说到q里惛_javascript里两个相{操作符==?==
//==只针对基本的数据cdQ不对ObjectQ如果待比较的两个值都是Object的话
//q回falseQ除非是同一个对象，如果有有一个是Object的话Q将之{化ؓ和另外一个值相同类型的?br />//然后q行比较?==则要求两者类型也相同?br />var b = new String("x");
alert(a == b);//true
alert(a===b);//false 因ؓcd不同
alert(typeof a);//string
alert(typeof b);//object
a = {};
//查看cd的方法是typeofQ可以写成typeof a也可以写成typeof(a)
//参看typeof的说?br />alert(typeof a);//object
alert(typeof true);//boolean
alert(typeof(typeof true))//string
typeof的执行逻辑
| Undefined | "undefined" |
| Null | "object" |
| Boolean | "boolean" |
| Number | "number" |
| String | "string" |
| Object (native and doesn’t implement [[Call]]) | "object" |
| Object (native and implements [[Call]]) | "function" |
| Object (host) | Implementation-dependent |
从上面可以看到对于ObjectcdQtypeofҎ值的情况q回object或function?br />Q注Q所?/span>implement [[Call]]我的理解是指是否可以作为函数调?span style="FONT-WEIGHT: bold" dragover="true">Qnative有两U一U是内置的如ArrayQDate{另一U是用户自己定义的。除此之外就是host即javascript的宿L供的一些对象。）
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