从头开始学JavaScript (三)——数据类型

原文:从头开始学JavaScript (三)——数据类型

一、分类

基本数据类型:undefined、null、string、Boolean、number

复杂数据类型:object

object的属性以无序的名称和值对的形式 (name : value) 来定义

二、详解

1、undefined:undefined类型只有一个值:undefined,在使用var对变量进行声明但未初始化时,这个变量的值就是undefined.

包含undefined值的变量与尚未定义的变量是不一样的,以下这个例子可以说明:

    var demo1;//声明但未初始化
alert(demo1);//undefined
alert(demo2);//报错,demo2 is not defined 

2、null:null类型只有一个值:null,从逻辑角度来看,null值表示一个空对象指针.

如果定义的变量准备在将来用于保存对象,那么最好将该变量初始化为null而不是其他值。这样一来,只要直接检测null值就可以知道相应的变量是否已经保存了一个对象的引用了,例如:

    if(car != null)
{
//对car对象执行某些操作
}

  实际上,undefined值是派生自null值的,因此ECMA-262规定对它们的相等性测试要返回true。

  alert(undefined == null); //true

  尽管null和undefined有这样的关系,但它们的用途完全不同。无论在什么情况下都没有必要把一个变量的值显式地设置为undefined,可是同样的规则对null却不适用。换句话说,只要意在保存对象的变量还没有真正保存对象,就应该明确地让该变量保存null值。这样做不仅可以体现null作为空对象指针的惯例,而且也有助于进一步区分null和undefined。

3、Boolean:Boolean类型有两个值:true和false,这两个值与数字值不是一回事,因此true不一定等于1,而false也不一定等于0。

需要注意的是:Boolean类型的字面值是区分大小写的,也就是说,True和False(以及其他形式的大小写混合)都不是Boolean值,只是标识符。

 虽然Boolean类型的字面值只有两个,但JavaScript中所有类型的值都有与这两个Boolean值等价的值。要将一个值转换为其对应的Boolean值,可以调用类型转换函数Boolean(),例如:

    var message = 'Hello World';
var messageAsBoolean = Boolean(message);

  在这个例子中,字符串message被转换成了一个Boolean值,该值被保存在messageAsBoolean变量中。可以对任何数据类型的值调用Boolean()函数,而且总会返回一个Boolean值。至于返回的这个值是true还是false,取决于要转换值的数据类型及其实际值。下表给出了各种数据类型及其对象的转换规则。

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" alt="">  

这些转换规则对理解流控制语句(如if语句)自动执行相应的Boolean转换非常重要,例如:

var message = 'Hello World';
if(message)//相当于if(Boolean(message)==true)
{
alert("Value is true");//Value is true
}

由于存在这种自动执行的Boolean转换,因此确切地知道在流控制语句中使用的是什么变量至关重要。

4、number:整数和浮点

4.1 整数:在进行计算时,所有的八进制和十六进制数都会被换算成十进制

4.2 浮点:浮点数值的最高精度是17位,因此在算数计算时其精度远远不如整数,例如:0.1+0.2的结果不是0.3,而是0.30000000000000004.例如:

a=0.2;
b=0.1;
if(a+b==0.3){
alert("hello");
}
else{
alert("hi");
}

结果会弹出“hi”,因此不要测试某个特定的浮点数值。

4.3 NaN:非数值 Not a Number,这个数值用于表示一个本来要返回数值的操作数未返回数值的情况(这样就不会抛出错误了)。

NaN本身有两个非同寻常的特点。首先,任何涉及NaN的操作(例如NaN/10)都会返回NaN,这个特点在多步计算中有可能导致问题。其次,NaN与任何值都不相等,包括NaN本身。例如:

alert(NaN == NaN);    //false

  JavaScript中有一个isNaN()函数,这个函数接受一个参数,该参数可以是任何类型,而函数会帮我们确定这个参数是否“不是数值”。isNaN()在接收一个值之后,会尝试将这个值转换为数值。某些不是数值的值会直接转换为数值,例如字符串”10“或Boolean值。而任何不能被转换为数值的值都会导致这个函数返回true。例如:

从头开始学JavaScript (三)——数据类型
    alert(isNaN(NaN));    //true
alert(isNaN(10)); //false(10是一个数值)
alert(isNaN("10")); //false(可能被转换为数值10)
alert(isNaN("blue")); //true(不能被转换为数值)
alert(isNaN("bule123")); //ture(不能被转换为数值)
alert(isNaN(true)); //false(可能被转换为数值1)
从头开始学JavaScript (三)——数据类型

  有3个函数可以把非数值转换为数值:Number()、parseInt()和parseFloat()。第一个函数,即转型函数Number()可以用于任何数据类型,而另外两个函数则专门用于把字符串转换成数值。这3个函数对于同样的输入会返回不同的结果。

  Number()函数的转换规则如下:

  ● 如果是Boolean值,true和false将分别被替换为1和0

  ● 如果是数字值,只是简单的传入和返回

  ● 如果是null值,返回0

  ● 如果是undefined,返回NaN

  ● 如果是字符串,遵循下列规则:

   ○ 如果字符串中只包含数字,则将其转换为十进制数值,即”1“会变成1,”123“会变成123,而”011“会变成11(前导的0被忽略)

   ○ 如果字符串中包含有效的浮点格式,如”1.1“,则将其转换为对应的浮点数(同样,也会忽略前导0)

   ○ 如果字符串中包含有效的十六进制格式,例如”0xf“,则将其转换为相同大小的十进制整数值

   ○ 如果字符串是空的,则将其转换为0

   ○ 如果字符串中包含除了上述格式之外的字符,则将其转换为NaN

  ● 如果是对象,则调用对象的valueOf()方法,然后依照前面的规则转换返回的值。如果转换的结果是NaN,则调用对象的toString()方法,然后再依次按照前面的规则转换返回的字符串值。

    var num1 = Number("Hello World");    //NaN
var num2 = Number(""); //0
var num3 = Number("000011"); //11
var num4 = Number(true); //1

  由于Number()函数在转换字符串时比较复杂而且不够合理,因此在处理整数的时候更常用的是parseInt()函数,而处理浮点数的时候常用parseFloat()函数,详见:http://www.cnblogs.com/yxField/p/4167954.html

5、String

String类型用于表示由零或多个16位Unicode字符组成的字符序列,即字符串。字符串可以由单引号(')或双引号(")表示。

    var str1 = "Hello";
var str2 = 'Hello';

  任何字符串的长度都可以通过访问其length属性取得

  alert(str1.length);        //输出5

  要把一个值转换为一个字符串有两种方式。第一种是使用几乎每个值都有的toString()方法。

    var age = 11;
var ageAsString = age.toString(); //字符串"11"
var found = true;
var foundAsString = found.toString(); //字符串"true"

  数值、布尔值、对象和字符串值都有toString()方法。但null和undefined值没有这个方法。

  多数情况下,调用toString()方法不必传递参数。但是,在调用数值的toString()方法时,可以传递一个参数:输出数值的基数。

从头开始学JavaScript (三)——数据类型
    var num = 10;
alert(num.toString()); //"10"
alert(num.toString(2)); //"1010"
alert(num.toString(8)); //"12"
alert(num.toString(10)); //"10"
alert(num.toString(16)); //"a"
从头开始学JavaScript (三)——数据类型

  通过这个例子可以看出,通过指定基数,toString()方法会改变输出的值。而数值10根据基数的不同,可以在输出时被转换为不同的数值格式。

  在不知道要转换的值是不是null或undefined的情况下,还可以使用转型函数String(),这个函数能够将任何类型的值转换为字符串。String()函数遵循下列转换规则:

  ● 如果值有toString()方法,则调用该方法(没有参数)并返回相应的结果

  ● 如果值是null,则返回"null"

  ● 如果值是undefined,则返回”undefined“

6、object

 对象其实就是一组数据和功能的集合。对象可以通过执行new操作符后跟要创建的对象类型的名称来创建。而创建Object类型的实例并为其添加属性和(或)方法,就可以创建自定义对象。

  var o = new Object();

  object类型所具有的任何属性和方法也同样存在于更具体的对象中,Object的每个实例都具有下列属性和方法:

  ● constructor(构造函数)——保存着用于创建当前对象的函数

  ● hasOwnProperty(propertyName)——用于检查给定的属性在当前对象实例中(而不是在实例的原型中)是否存在。其中,作为参数的属性名(propertyName)必须以字符串形式指定(例如:o.hasOwnProperty("name"))

  ● isPrototypeOf(object)——用于检查传入的对象是否是另一个对象的原型

  ● propertyIsEnumerable(propertyName)——用于检查给定的属性是否能够使用for-in语句来枚举

  ● toString()——返回对象的字符串表示

  ● valueOf()——返回对象的字符串、数值或布尔值表示。通常与toString()方法的返回值相同。

 三、小测试
typeof(NaN)
typeof(Infinity)
typeof(null)
typeof(undefined)
很多面试都会问到上面几个小问题哒~~
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