You can safely use the typeof operator on undefined variables.
If it has been assigned any value, including null, typeof will return something other than undefined. typeof always returns a string.
Therefore
if (typeof maybeObject != “undefined”) {
alert(“GOT THERE”);
}
There are a lot of half-truths here, so I thought I make some things clearer.
Actually you can’t accurately tell if a variable exists (unless you want to wrap every second line into a try-catch block).
The reason is Javascript has this notorious value of undefined which strikingly doesn’t mean that the variable is not defined, or that it doesn’t exist undefined !== not defined
var a;
alert(typeof a); // undefined (declared without a value)
alert(typeof b); // undefined (not declared)
So both a variable that exists and another one that doesn’t can report you the undefined type.
As for @Kevin’s misconception, null == undefined. It is due to type coercion, and it’s the main reason why Crockford keeps telling everyone who is unsure of this kind of thing to always use strict equality operator === to test for possibly falsy values. null !== undefined gives you what you might expect. Please also note, that foo != null can be an effective way to check if a variable is neither undefined nor null. Of course you can be explicit, because it may help readability.
If you restrict the question to check if an object exists, typeof o == “object” may be a good idea, except if you don’t consider arrays objects, as this will also reported to be the type of object which may leave you a bit confused. Not to mention that typeof null will also give you object which is simply wrong.
The primal area where you really should be careful about typeof, undefined, null, unknown and other misteries are host objects. They can’t be trusted. They are free to do almost any dirty thing they want. So be careful with them, check for functionality if you can, because it’s the only secure way to use a feature that may not even exist.