Question

Basically two questions, both pertaining to the behavior of undefined.

Q1 Why is it that this fails with a reference error:

function hello(a){
      return true;  
      }  
//var abc = undefined;
hello(abc);

but this doesn’t:

function hello(a){
      return true;  
      }  
var abc = undefined;
hello(abc);

Q2 Why is it that this is a valid assignment:

undefined = 10;

But these aren’t:

null = 10;
"hello" = 10;
10 = 10;

Thanks for your help.

Answer 1

In the first case you are not defining the key 'abc' on the global object and thus you are having a reference error trying to access it. It is something like 'Name error' in ruby and python saying that an identifier is undefined.

These two are equivalent and they both define the 'abc' key on the global object, in your case this is window.

var abc = undefined;
var abc;

Defining undefined as an identifier works. It defines the window.undefined key not the undefined type. This means that:

undefined = 10;
var a;
alert(a); // Yeilds undefined
alert(window.undefined); // Yeilds 10

In order to perform undefined check, check the type of the identifier:

if (typeof identifier === 'undefined') // Performs undefined check

Because:

if (identifier === undefined) // Gives us unexpected results 
// if undefined has been assigned to

In addition undefined is not a primitive. While 0, 'string' are primitives. null is a special singleton object as pointed out in the comments. Those are not identifiers but actual values so you cannot assign to them by specification.

Update regarding NaN:

NaN is slightly different because it is a primitive (number). Trying to do the same with NaN yeilds:

>> typeof NaN
"number"
>> NaN = 'a'
"a"
>> 0 === NaN
false
>> 'a' === NaN
false
>> typeof NaN
"number"
>> NaN
NaN