> I’m writing a test case to determine whether a javascript variable exists.
>
> Any thoughts?
Well, first of all, in JavaScript null is an object. There’s another
value for things that don’t exist, undefined. The DOM returns null for
almost all cases where it fails to find some structure in the
document, but in JavaScript itself undefined is the value used.
Second, no, they are not directly equivalent.
If you really want to check for null, do:
if (null == yourvar) // with casting
if (null === yourvar) // without casting
If you want to check if a variable exist
if (typeof yourvar != ‘undefined’) // Any scope
if (window[‘varname’] != undefined) // Global scope
if (window[‘varname’] != void 0) // Old browsers
If you know the variable exists but don’t know if there’s any value
stored in it:
if (undefined != yourvar)
if (void 0 != yourvar) // for older browsers
If you want to know if a member exists independent of whether it has
been assigned a value or not:
if (‘membername’ in object) // With inheritance
if (object.hasOwnProperty(‘membername’)) // Without inheritance
If you want to to know whether a variable autocasts to true:
if(variablename)
I probably forgot some method as well…
—
David “liorean” Andersson
