undefined means a variable has been declared but has not yet
been assigned a value. On the other hand, null is an assignment value. It can
be assigned to a variable as a representation of no value.
Also, undefined and null are two distinct types: undefined
is a type itself (undefined) while null is an object.
Unassigned variables are initialized by JavaScript with a
default value of undefined. JavaScript never sets a value to null. That must be
done programmatically.
