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

Answer 1

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
}