How do you check undefined ?

How do you check if a variable is undefined in JavaScript ?

Solution 1:

(typeof x === "undefined")

It turns out that, checking x directly instead of passing it to a typeof operator and then doing a string match.

Solution 2:

( x === undefined )

Still, this might fall into the bug where someone redefines undefined to something else.

undefined = 1 // BOOM!

So, the best way to check for undefined is to use the void operator which is defined as

"The void operator evaluates the given expression and then returns undefined."

Solution 3:

(x === void 0)

The void operator allows inserting expressions that produce side effects into places where an expression that evaluates to undefined is desired.