undefined in JavaScript

gistfile1.js

// Alternative 1 - unsafe
function isUndefined(val) {
    // undefined is a global variable, not a language keyword or a constant
    // somebody could have reassigned it to, say, "foobar"!
    //
    return val === undefined;  
}

// Alternative 2 - safe 
function isUndefined(val) {
   // the local variable undefined has the value |undefined| (a kind of special
   // value in the ECMAScript specification) because it isn't initialized. 
   //
   var undefined 
   return val === undefined;
}

// Alternative 3 - safe 
function isUndefined(val) {
   // void(0) is an expression evaluating to |undefined| 
   //
   return val === void(0);
}