undefined vs null in JavaScript

undefined

undefined is a primitive value automatically assigned to variables that have just been declared, or to formal arguments for which there are no actual arguments.

let x //create a variable but assign it no value

console.log(x's value is ${x}); //logs "x's value is undefined"

null

In computer science, a null value represents a reference that points, generally intentionally, to a nonexistent or invalid object or address. The meaning of a null reference varies among language implementations.

In JavaScript, null is marked as one of the primitive values, because its behavior is seemingly primitive. However, when using the typeof operator, it returns "object".

console.log(typeof null) // "object"

This is considered a bug, but one which cannot be fixed because it will break too many scripts.

primitive value

In JavaScript, a primitive (primitive value, primitive data type) is data that is not an object and has no methods or properties. There are 7 primitive data types:

string
number
bigint
boolean
symbol
undefined
null