Why doesn't the toString method work when displaying the result?

Question

JavaScript

Why am I getting the result of a function in the console and not a calculation?

function func(n1) {
    return function foo(n2) {
        foo.valueOf = () => n1 + n2
        foo.toString = () => n1 + n2
        return func(n1 + n2)
    }
}

const fo = func(3)
console.log(fo(10))

Answer 1

function func(n1) { 
  const foo = n2 => func(n1 + n2)
  foo.toString = () => n1
  return foo;
}

const fo = func(3)
console.log(fo(10))