Types: undefined and null

In TypeScript, undefined and null are types themselves; this means that undefined is a type (undefined) and null is a type (null). Confusing? undefined and null cannot be  type  variables; they can only be assigned as values to variables.

They are also different: a null variable means that a variable was set to null, while an undefined variable has no value assigned.

let A = null;
    console.log(A) // null
    console.log(B) // undefined