How can I check for "undefined" in JavaScript? [duplicate]

If you are interested in finding out whether a variable has been declared regardless of its value, then using the in operator is the safest way to go. Consider this example:

// global scope
var theFu; // theFu has been declared, but its value is undefined
typeof theFu; // "undefined"

But this may not be the intended result for some cases, since the variable or property was declared but just not initialized. Use the in operator for a more robust check.

"theFu" in window; // true
"theFoo" in window; // false

If you are interested in knowing whether the variable hasn't been declared or has the value undefined, then use the typeof operator, which is guaranteed to return a string:

if (typeof myVar !== 'undefined')

Direct comparisons against undefined are troublesome as undefined can be overwritten.

window.undefined = "foo";
"foo" == undefined // true

As @CMS pointed out, this has been patched in ECMAScript 5th ed., and undefined is non-writable.

if (window.myVar) will also include these falsy values, so it's not very robust:

false
0
""
NaN
null
undefined

Thanks to @CMS for pointing out that your third case - if (myVariable) can also throw an error in two cases. The first is when the variable hasn't been defined which throws a ReferenceError.

// abc was never declared.
if (abc) {
    // ReferenceError: abc is not defined
} 

The other case is when the variable has been defined, but has a getter function which throws an error when invoked. For example,

// or it's a property that can throw an error
Object.defineProperty(window, "myVariable", { 
    get: function() { throw new Error("W00t?"); }, 
    set: undefined 
});
if (myVariable) {
    // Error: W00t?
}

I personally use

myVar === undefined

Warning: Please note that === is used over == and that myVar has been previously declared (not defined).

I do not like typeof myVar === "undefined". I think it is long winded and unnecessary. (I can get the same done in less code.)

Now some people will keel over in pain when they read this, screaming: "Wait! WAAITTT!!! undefined can be redefined!"

Cool. I know this. Then again, most variables in Javascript can be redefined. Should you never use any built-in identifier that can be redefined?

If you follow this rule, good for you: you aren't a hypocrite.

The thing is, in order to do lots of real work in JS, developers need to rely on redefinable identifiers to be what they are. I don't hear people telling me that I shouldn't use setTimeout because someone can

window.setTimeout = function () {
    alert("Got you now!");
};

Bottom line, the "it can be redefined" argument to not use a raw === undefined is bogus.

(If you are still scared of undefined being redefined, why are you blindly integrating untested library code into your code base? Or even simpler: a linting tool.)

Also, like the typeof approach, this technique can "detect" undeclared variables:

if (window.someVar === undefined) {
    doSomething();
}

But both these techniques leak in their abstraction. I urge you not to use this or even

if (typeof myVar !== "undefined") {
    doSomething();
}

Consider:

var iAmUndefined;

To catch whether or not that variable is declared or not, you may need to resort to the in operator. (In many cases, you can simply read the code O_o).

if ("myVar" in window) {
    doSomething();
}

But wait! There's more! What if some prototype chain magic is happening…? Now even the superior in operator does not suffice. (Okay, I'm done here about this part except to say that for 99% of the time, === undefined (and ****cough**** typeof) works just fine. If you really care, you can read about this subject on its own.)

2020 Update

One of my reasons for preferring a typeof check (namely, that undefined can be redefined) became irrelevant with the mass adoption of ECMAScript 5. The other, that you can use typeof to check the type of an undeclared variable, was always niche. Therefore, I'd now recommend using a direct comparison in most situations:

myVariable === undefined

Original answer from 2010

Using typeof is my preference. It will work when the variable has never been declared, unlike any comparison with the == or === operators or type coercion using if. (undefined, unlike null, may also be redefined in ECMAScript 3 environments, making it unreliable for comparison, although nearly all common environments now are compliant with ECMAScript 5 or above).

if (typeof someUndeclaredVariable == "undefined") {
    // Works
}

if (someUndeclaredVariable === undefined) { 
    // Throws an error
}

You can use typeof, like this:

if (typeof something != "undefined") {
    // ...
}