If you try to use a non-existing variable, you'll get an error:

> foo;
ReferenceError: foo is not defined

Using the typeof operator on a non-existing variable is not an error. You get the string "undefined" back:

> typeof foo;
"undefined"

If you declare a variable without giving it a value, this is, of course, not an error. But, the typeof still returns "undefined":

> var somevar;
> somevar;
> typeof somevar;
"undefined"

This is because when you declare a variable without initializing it, JavaScript automatically initializes it with the value undefined:

> var somevar;
> somevar === undefined;
true

The null value, on the other hand, is not assigned by JavaScript behind the scenes; it's assigned by your code:

> var somevar = null;
null
> somevar;
null
> typeof somevar;
"object"

Although the difference between null and undefined is small, it could be critical at times. For example, if you attempt an arithmetic operation, you get different results:

> var i = 1 + undefined;
> i;
NaN
> var i = 1 + null;
> i;
1

This is because of the different ways null and undefined are converted to the other primitive types. The following examples show the possible conversions: