Примеры Undefined

typeof a; //"undefined"

var a;
typeof a; //"undefined"

window.b;
typeof window.b; //"undefined"

var c = (function() {})();
typeof c; //"undefined"

var d = (function(e) {return e})();
typeof d; //"undefined"

var f = 2;
f = undefined; //re-assigning to undefined (variable)
typeof f; //"undefined"

if (typeof console != "undefined") {
    console.log(new Date());
}