I know the best way to check if a variable is undefined is
if ( typeof( something ) == "undefined")
What I don't understand is when is a variable undefined and when is an object undefined. For instance when I console.log() the a variable I get
Object {detailedView: undefined}
My question is why I am getting this and not a plain undefined.
a isn't undefined. It's defined. It's an object. That object just so happens to contain a property that is undefined, but the variable itself isn't undefined.
something isn't defined, so it's undefined.
There are two things here: the "value" undefined, which is a value that a variable can point to when it has been declared but not assigned anything yet (i.e. var a;), or when it's been assigned something that doesn't actually have a value or exists, and there is the string "undefined", which is a string similar to "monkey" or "collywobble".
In your conditional, you're testing to see what the result of the typeof operator is, which is a string "object" or "function", or in this case "undefined". Those are just words:
if(typeof a == "undefined")
is the same as testing
if(a == undefined)
So, when you console.log the actual object, you'll see it has a value undefined, rather than being a string "undefined".
I believe you're asking about the difference between "undefined variables" (variables never declared) and "undefined values" (the value undefined inside a variable or property).
You define/declare a variable with the var keyword:
var myVariable;
If you just do that, the value of that variable is undefined:
console.log(myVariable); // undefined
If you don't declare a variable, you cannot use it:
console.log(myOtherVariable); // throws a ReferenceError
... except in typeof:
typeof myOtherVariable == "undefined"; // true
