JavaScript Difference Between null and undefined

In JavaScript, undefined means a variable has been declared but has not yet been assigned a value, such as:

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.
Unassigned variables are initialized by JavaScript with a default value of undefined.
JavaScript never sets a value to null. That must be done programmatically. As such, null can be a useful debugging tool. If a variable is null, it was set in the program, not by JavaScript.
null values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: 0
String: "null"

undefined values are evaluated as follows when used in these contexts:

Boolean: false
Numeric: NaN
String: "undefined"