Difference between null and undefined (explained in 5 seconds)

undefined means a variable has been declared but has not yet been assigned a value.

var TestVar;
alert(TestVar); //shows undefined
alert(typeof TestVar); //shows undefined

null is an assignment value. It can be assigned to a variable as a representation of no value.

var TestVar = null;
alert(TestVar); //shows null
alert(typeof TestVar); //shows object

From the preceding examples, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.