What is the difference between null and undefined in Javascript?

What is null?

• The null value in javascript represents a non-existent or empty value.
• Null must be assigned.

let a = null;
//output: null
console.log(a);

• Null is one of the falsy values in javascript. It means it is treated as false in any boolean operation in javascript.

//output: false
console.log(Boolean(null))

• If you check the type of null using typeof operator, it will return the object. It is a bug in Javascript code and hasn't been removed because a lot of libraries and code depends on it.

let a = null;
let nullType = typeof(a);
//output: "object"
console.log(nullType);

What is undefined?

• Undefined in javascript means that a variable has been declared, but no value is assigned to it.

let newVar;
//output: "undefined"
console.log(newVar);

• Undefined is also one of the falsy values in javascript. It means it is treated as false in any boolean operation in javascript.

//output: false
console.log(Boolean(undefined))

• You can also assign undefined value to a variable explicitly.

var newVar = undefined;
//output: false
console.log(newVar);

• typeof undefined in javascript is undefined. You can use typeof operator to check if a variable is undefined in javascript.

var newVar = undefined;
//output: true
console.log(typeof(newVar) === 'undefined');

Operations between null and undefined

//"object" (for legacy reasons)
typeof null          

//"undefined"
typeof undefined     

//false as both are of different types
null === undefined   

//true
null  == undefined      

Related Article: Javascript Data Types Explained