The void operator will evaluate to undefined regardless of its operand. Its operand can be any valid reference or expression:

void 1; // => undefined
void null; // => undefined
void [1, 2, 3]; // => undefined

It doesn't have many uses nowadays, although void 0 is sometimes used as an idiom for undefined either for succinctness or to avoid issues in legacy environments where undefined was an untrusted mutable value.