Condition Always true when checking for "undefined" in Javascript

Vimalsoft Pty Ltd

Good Day all i am reluctantly maintaining a Classic Asp application that will be re-written in few months to come. i have an issue where i have a Code like this

if(document.frmEntry.optMethod != "undefined")
                    {
                        if (document.frmEntry.optMethod.selectedIndex == 0) {
                            strPrompt=strPrompt+'Please specify the Valuation Method\\n';
                            if ( objFocus == null ) {
                                objFocus = document.frmEntry.optMethod;
                            }
                        }
                    }

my problem here is that even if document.frmEntry.optMethod is not equal to "undefined" the code still go into the condition as if it was true. i have attached a proof in my debuger. http://www.vetauinvest.com/Example/IE_DEBUGGER.png[^] Thanks

Vuyiswa Maseko, Spoted in Daniweb-- Sorry to rant. I hate websites. They are just wierd. They don't behave like normal code. C#/VB.NET/ASP.NET/SQL7/2000/2005/2008 http://www.vuyiswamaseko.com vuyiswa[at]dotnetfunda.com http://www.Dotnetfunda.com

Blikkies

Try using a Falsy check. The following values are always falsy: false 0 (zero) "" (empty string) null undefined NaN (a special Number value meaning Not-a-Number!) Your code should look like as follows:

if(!document.frmEntry.optMethod)
{
if (document.frmEntry.optMethod.selectedIndex == 0) {
strPrompt=strPrompt+'Please specify the Valuation Method\n';
if ( objFocus == null ) {
objFocus = document.frmEntry.optMethod;
}
}
}

Kornfeld Eliyahu Peter

That should be undefined and NOT "undefined"!!!

I'm not questioning your powers of observation; I'm merely remarking upon the paradox of asking a masked man who he is. (V)

Graham Breach

The "undefined" shown in the debugger is not a string, so you can't compare it as one. To really test if your value is undefined, you can use this:

if(typeof document.frmEntry.optMethod != 'undefined')

The typeof operator returns the type of the argument as a string.

Kornfeld Eliyahu Peter

I can't see why use undefined as string - with or without typeof!? In JavaScript undefined is a value property of the global namespace and yous should use it according to it...

I'm not questioning your powers of observation; I'm merely remarking upon the paradox of asking a masked man who he is. (V)

Kornfeld Eliyahu Peter

You are very true about 'false' check, but sometimes it is a good practice to write full code and explicitly state against what are you want to check. It's of course only a matter of maintainability/readability...

I'm not questioning your powers of observation; I'm merely remarking upon the paradox of asking a masked man who he is. (V)

Vimalsoft Pty Ltd

Thank you , this info is enough to solve the issue. thanks again

Vuyiswa Maseko, Spoted in Daniweb-- Sorry to rant. I hate websites. They are just wierd. They don't behave like normal code. C#/VB.NET/ASP.NET/SQL7/2000/2005/2008 http://www.vuyiswamaseko.com vuyiswa[at]dotnetfunda.com http://www.Dotnetfunda.com

Graham Breach

The typeof for an undefined value is always "undefined". The value undefined being in the global namespace opens up some possible problems:

function madness() {
// override with local variable
var a = 'fish', undefined = 'fish';

if(a === undefined) {
// both 'fish', so here we are
}
}

function mistake() {
var a = 'fish';

// common mistake, using '=' instead of '=='
if(undefined = a) {
// 'undefined' is not a constant or keyword, so the whole
// expression is valid and will be true
}
}

I know these are pretty unlikely scenarios, but better safe than sorry. In earlier browsers "undefined" was not a testable value so expressions like (a == undefined) would produce the helpful message "undefined is undefined".

jkirkerx

Vuyiswa Maseko wrote:

if(document.frmEntry.optMethod != "undefined")

I would of wrote it like this. I'm pretty sure undefined is like NaN, it's a recognized parameter.

if(document.frmEntry.optMethod !== undefined) {

}

Kornfeld Eliyahu Peter

Graham Breach wrote:

The typeof for an undefined value is always "undefined".

How that?

var undefined = 'fish';
alert(typeof undefined); // string!!!

I'm not questioning your powers of observation; I'm merely remarking upon the paradox of asking a masked man who he is. (V)

Graham Breach

Yes. If you define undefined as a string variable, then it is no longer undefined and things get confusing. This is why I was testing the type of the value against the string "undefined" - the string is a constant.

Tushar Guru

You can try with remove ("") double quotes OR use ('') single quotes.