AW: AW: [thelist] javascript detecting if function/var is already defined

christof.hoeke at christof.hoeke at
Fri Nov 28 05:48:09 CST 2003

Just a test:

var x = 'x';
     return [x,
         x == undefined,
         typeof x];
})(); // => [ , true, 'undefined' ]

Nope, that does not seem to work. The reason? Have a look at the 
following list:
1. Determine if member exists on an object:
//(it may exists but be undefined)
   'x' in oParent // => boolean
2. Check what type the variable is:
   typeof x // => string:
// 'undefined'|'object'|'number'|'boolean'|'function'|'string'
// ('undefined' is the result if the variable is not defined,
// independently of whether it exists or not)
3. Determine if the variable is undefined
// (assumes the existense of the variable in the first place)
   x != undefined // => boolean
4. Determine if variable autocasts to true, variants
   Boolean(x) // => boolean
   !x // => boolean (inverted)
   x || false // => anything (but false if it autocasts to false)
liorean <mailto:liorean at>

-----Ursprüngliche Nachricht-----
Von: liorean [mailto:liorean at]
Gesendet: Freitag, 28. November 2003 12:35
An: thelist at
Betreff: Re: AW: [thelist] javascript detecting if function/var is
already defined

however it does seem to work on the top level:

	a = 2
	var a
	if (a == undefined) alert("a is undefined");
	else alert("a = " + a)

	var b
	if (b == undefined) alert("b is undefined");
	else alert("b = " + b)

=> 	a = 2
and 	b = undefined

but i have to admit that typeof seems more right.


More information about the thelist mailing list