What is the difference between null and undefined in JavaScript?

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.

null === undefined // false
null == undefined // true
null === null // true

and

null = 'value' // ReferenceError
undefined = 'value' // 'value'