What exactly is the difference between undefined and void 0 ?
Which is preferred and why?
The difference is that it is sometimes possible to overwrite the value of undefined. However, void anything always returns real undefined (as long as anything finishes evaluating).
ECMAScript 5 limited this problem by making undefined at global scope read-only (ECMA-262 Ed. 5.1, §15.1.1.3). Nevertheless, undefined remains an ordinary identifier subject to scope lookup and shadowing, so potentially it can still be overridden in a nested scope:
undefined = 42;
console.log(0, typeof void 0); // "undefined"
console.log(1, typeof undefined); // "undefined" in ES5 and later
// ("number" earlier)
{
let undefined = 42;
console.log(2, typeof void 0); // "undefined"
console.log(3, typeof undefined); // "number"
}
(function (undefined) {
undefined = 42;
console.log(4, typeof void 0); // "undefined"
console.log(5, typeof undefined); // "number"
})();