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
Both of them are representing a value of a variable with no value
AND null doesn’t represent a string that has no value
Like
var a = ”;
console.log(typeof a); // string
var a = ” ;
console.log(a == null); //false
console.log(a == undefined); // false
Now if
var a ;
console.log(a == null); //true
console.log(a == undefined); //true
But
var a;
console.log(a === null); //false
console.log(a === undefined); // true
So each one has it own way to use
undefined use it to compare the variable data type
null use it to empty a value of a variable
var a = ‘javascript’;
a = null ; // will change the type of variable “a” from string to undefined
Now it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.
