测试以查看变量是否存在... [英] Testing to see if a variable exists...
问题描述
Hey Folks，
任何人都知道如何在javascript中测试变量的存在？
我正在寻找相当于PHP的IsSet（）。我遇到了麻烦
找到它。
thx
-
im
美国爱国者法案是美国历史上最不爱国的行为。
Hey Folks,
Anyone know how to test for the existance of a variable in javascript?
I''m looking for the equivalent of IsSet() from PHP. I''m having trouble
finding it.
thx
--
i.m.
The USA Patriot Act is the most unpatriotic act in American history.
推荐答案
Ivan Marsh写道：
Ivan Marsh wrote:
任何人都知道如何在javascript中测试变量的存在？
Anyone know how to test for the existance of a variable in javascript?
检查''typeof ''运营商：
< script type =" text / javascript">
alert（typeof foo）;
if（typeof foo ==" undefined"）{
//做东西......
}
< / script>
学习这个属性时，最好在不同的用户代理中迭代许多
js / html类型，以检查返回的内容价值。
你有时可能会感到惊讶（尤其是js中的null和Mozilla / IE中的HTML
元素）。
HTH
是的。
Check the ''typeof'' operator:
<script type="text/javascript">
alert( typeof foo );
if(typeof foo == "undefined") {
// do stuff...
}
</script>
It''s a good idea, when learning this property, to iterate over many
js/html types, in different user agents, to check the returned value.
You might be surprised sometimes (especially for "null" in js, and HTML
elements in Mozilla/IE).
HTH
Yep.
Ivan Marsh写道：
Ivan Marsh wrote:
任何人都知道如何在javascript中测试变量的存在？
我正在寻找PHP的等效IsSet（）。我很难找到它。
Anyone know how to test for the existance of a variable in javascript?
I''m looking for the equivalent of IsSet() from PHP. I''m having trouble
finding it.
与PHP中的isset不完全相同但JavaScript方式通常是
if（typeof varname！=''undefined''）{
//使用varName
}
然而这种方法并不是允许您区分
变量是否尚未声明或已声明但尚未初始化，
如果您声明
var varname ;
然后
typeof varname
仍然是'undefined''。但这通常不会打扰你，因为你只需要b $ b想要检查变量中是否存储了有用的东西。
-
Martin Honnen
http://JavaScript.FAQTs.com /
Not quite the same as isset in PHP but the JavaScript way is usually
if (typeof varname != ''undefined'') {
// use varName
}
However that approach doesn''t allow you to distinguish whether a
variable has not been declared or has been declared but not intialized,
that is if you declare
var varname;
then
typeof varname
is still ''undefined''. But that usually doesn''t bother you as you only
want to check whether something useful is stored in the variable.
--
Martin Honnen
http://JavaScript.FAQTs.com/
" Ivan Marsh"写于2003年11月11日：
"Ivan Marsh" wrote on 11/11/2003:
嘿民众，
任何人都知道如何测试
javascript中变量的存在？<我正在寻找PHP中等效的IsSet（）。我找不到
麻烦。
Hey Folks,
Anyone know how to test for the existance of a variable in javascript?
I''m looking for the equivalent of IsSet() from PHP. I''m having trouble finding it.
使用此功能：
函数isSet（变量）
{
return（typeof（variable）！=''undefined''）;
}
如果定义了变量但尚未分配变量，它将起作用。对于
示例：
var a;
var b =''used'';
isSet（a）; // false
isSet（b）; // true
如果你想测试那些根本没有声明的变量，你需要直接测试，否则会发生错误。继续上面的
：
if（typeof（c）==''undefined''）
{
//这将在此示例中执行
// c未定义或尚未分配给...
}
其他
{
//已定义c ...
}
Mike
-
Michael Winter
M.Winter @ [no-spam] blueyonder.co.uk（删除[无垃圾邮件]回复）
Use this function:
function isSet( variable )
{
return( typeof( variable ) != ''undefined'' );
}
It will work if a variable is defined, but not assigned to yet. For
example:
var a;
var b = ''used'';
isSet( a ); // false
isSet( b ); // true
If you want to test for variables that have not been declared at all,
you have to test directly, or an error will occur. Continuing from
above:
if( typeof( c ) == ''undefined'' )
{
// this will execute in this example
// c is not defined or has not been assigned to...
}
else
{
// c has been defined...
}
Mike
--
Michael Winter
M.Winter@[no-spam]blueyonder.co.uk (remove [no-spam] to reply)
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