(tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) JavaScript中的BooleanQ你?jin)解多少Q?/font>
(tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng) (tng)马嘉?/font> (tng) (tng) (tng) (tng) (tng) (tng)2006-10-30
今天看了(jin)一?a href="ttp://www.aygfsteel.com/flyingis">Flyingis写的?a href="/flyingis/archive/2006/06/13/52484.html?CommentID=78102#Post">ECMAScript 基础》一文。在看到
Booleanc这一块内容的时候,随手写了(jin)几行代码Q问题出C(jin)^+^
注:(x)此文讨论的是JavaScript1.5
q是先来看一D非常简单的代码
//
codeQ?
var
(tng)x (tng)
=
(tng)
false
; (tng) (tng) (tng)
var
(tng)y (tng)
=
(tng)
true
;
//
codeQ?
var
(tng)xObject (tng)
=
(tng)
new
(tng)Boolean(
false
);
var
(tng)yObject (tng)
=
(tng)
new
(tng)Boolean(
true
);
//
codeQ?
var
(tng)resX (tng)
=
(tng)
new
(tng)Boolean(x);
var
(tng)resY (tng)
=
(tng)
new
(tng)Boolean(y);
//
codeQ?
var
(tng)resXObject (tng)
=
(tng)
new
(tng)Boolean(xObject);
var
(tng)resYObject (tng)
=
(tng)
new
(tng)Boolean(yObject);
//
codeQ?
alert(
"
resX (tng)= (tng)
"
(tng)
+
(tng)resX (tng)
+
(tng)
"
(tng)and (tng)resY (tng)= (tng)
"
(tng)
+
(tng)resY);
alert(
"
resXObject (tng)= (tng)
"
(tng)
+
(tng)resXObject (tng)
+
(tng)
"
(tng)and (tng)resYObject (tng)= (tng)
"
(tng)
+
(tng)resYObject);
你明白codeQ?~4 之间的区别么Q(你一定对codeQ?最为熟(zhn),q是我们最常用的)(j)
你知道codeQ? 的输出结果么Q(可以先想惻I在运行代码看l果Q如果不明白Q可以l看下去Q?br />
如果你已l开始头晕了(jin)Q那暂时不L?jin),看我下面的解释?br />如果你都知道?jin)(哇,高手啊,呵呵^+^Q,那我q有个问题需要你的解{,在此先谢q了(jin)?br />
正文Q?br />JavaScript中的Booleancd
1. (tng) (tng) (tng)我们所熟?zhn)?br />
var x = false;
var y = true;
q是我们大家都熟(zhn)的Q此时我们用的是原始的Boolean|the primitive Boolean values Qtrue和false
2. (tng) (tng) (tng)我们很少用到?/font>
var xObject = new Boolean(false);
var yObject = new Boolean(true);
此时我们声明?jin)一?font color="#0000ff">Boolean对象QBoolean对象是对Boolean值的一个封装?br />Boolean对象Q一个gؓ(f)true或false的Boolean对象?br />
原始的Boolean值和Boolean对象是有区别的,不要怺hQ引?font color="#0000ff">Core JavaScript 1.5 Reference中的一句话Q?font color="#a52a2a">Do not confuse the primitive Boolean values true and false with the true and false values of the Boolean object.
3. (tng) (tng) (tng)区别
区别Q?Q:(x) (tng) (tng) (tng)声明形式
区别Q?Q:(x) (tng) (tng) (tng)在条件语句(condition statementQ中
var (tng)x (tng)= (tng)false;
var (tng)xObject (tng)= (tng)new (tng)Boolean(false);
if(x)
(tng) (tng) (tng) (tng)alert("x (tng)= (tng)true");
else
(tng) (tng) (tng) (tng)alert("x (tng)= (tng)false");
if(xObject)
(tng) (tng) (tng) (tng)alert("xObject (tng)= (tng)" (tng)+ (tng)xObject (tng)+ (tng)", (tng)but (tng)in (tng)the (tng)condition (tng)statement, (tng)the (tng)xObject (tng)value (tng)is (tng)evaluated (tng)to (tng)true");
else
(tng) (tng) (tng) (tng)alert("xObject (tng)= (tng)false");
输出l果Q?br />
x (tng)= (tng)false
xObject (tng)= (tng)false, (tng) (tng)but (tng)in (tng)the (tng)condition (tng)statement, (tng)the (tng)xObject (tng)value (tng)is (tng)evaluated (tng)to (tng)true
当一个gؓ(f)false的Boolean对象攑ֈ条g语句当中的时候,Boolean对象的g(x)被当作true来计,但是它本w的值ƈ没有改变Q依然是false。很有趣Q我只知道现象,具体原因不清楚,不知道发明者出于什么地考虑。但是在JavaScript1.2或者更早的版本中却有所不同Q在条g语句Qcondition statementQ中QBoolean对象的计返回的是其对应的原始Boolean的倹{?br />
区别Q?Q:(x)初始化Boolean对象
声明一个Boolean对象Q如果我们用一个gؓ(f)false的Boolean对象对其q行初始化,则新的Boolean对象的gؓ(f)true
var (tng)xObject (tng)= (tng)new (tng)Boolean(false);
var (tng)resXObject (tng)= (tng)new (tng)Boolean(xObject);
alert("The (tng)resXObject (tng)value (tng)is (tng)" (tng)+ (tng)resXObject);
原始的Boolean值和Boolean对象区别如此之大Q所以切C者区别,不要h使用?br />
对于codeQ? 的输出结果,你现在明白是什么原因了(jin)吧?br />
上面的东东我也是今天遇到?jin)问题,查?jin)一些资料才知道Q学无止境啊。可是我的问题(sh)然没有解冟?br />我的问题是关于Boolean对象的初始化。代码如下,
var (tng)ob1 (tng)= (tng)new (tng)Boolean(false);
var (tng)ob2 (tng)= (tng)(ob1&&true);
var (tng)ob3 (tng)= (tng)(true&&ob1);
var (tng)ob4 (tng)= (tng)Boolean(ob1&&true);
var (tng)ob5 (tng)= (tng)Boolean(true&&ob1);
alert("ob2 (tng)= (tng)" (tng)+ (tng)ob2 (tng)+ (tng)" (tng)ob3 (tng)= (tng)" (tng)+ (tng)ob3 (tng)+ (tng)" (tng)ob4 (tng)= (tng)" (tng)+ (tng)ob4 (tng)+ (tng)" (tng)ob5 (tng)= (tng)" (tng)+ (tng)ob5);
l果Q?br />
ob2 (tng)= (tng)true (tng)ob3 (tng)= (tng)false (tng)ob4 (tng)= (tng)true (tng)ob5 (tng)= (tng)true
我对ob2 ?ob3的结果不太理解,Z么我只是更改?jin)ob1和true的顺序,l果d全不同了(jin)。ƈ且可以发玎ͼl果均等于操作符?amp;&”后面的倹{?br />
希望大家能告诉我Q?br />
问题已经解决Q?/font>
||是这栯的Q从W一个开始,遇到有意义的q回Q否则返回最后一个表辑ּQ注意不一定是Boolean|(j)Q?
&&是这栯的Q从W一个开始,遇到无意义的q回Q否则返回最后一个表辑ּQ注意同上)(j)Q?
!是这栯的Q?/font>对表辑ּ的值取非(注意不是对表辑ּQ?
什么是无意义呢Q如下六?0,null,undefined,"",false,NaN。除此,视ؓ(f)有意义?
new Boolean(),new Boolean(false)是同一个东西,׃它是一个对象,故是有意义的Q但其gؓ(f)falseQ所以,可以看ؓ(f)“有意义的false”,所以结果ؓ(f)最后一个表辑ּ的倹{?
谢谢
jeffjie
l箋(hu)学习(fn)^+^

]]>