null

null is a primitive data type in JavaScript. It indicates that a variable points to no object.

// count exists, but it has no type or value
let count = null

console.log(null)
// null

undefined

undefined is a primitive data type in JavaScript. It indicates that a variable has not been assigned any value.

// count has been defined, but not initialized
let count

console.log(count)
// undefined

Another way to think about null & undefined is as follows:

undefined: Used for unintentionally missing values

null: Used for intentionally missing values