How to add TypeScript types to destructured parameters using spread syntax?

Question

Ignore the fact that this is bad add function. It\'s a question about using array destructuring with spread syntax in TypeScript.

This is what I\'m tryi

Answer 1

My bad, the answer is as simple as:

const add = ([x, ...xs]: number[]) => {
  if (x === undefined)
    return 0
  else
    return x + add(xs)
}

console.log(add([1, 2, 3])); //=> 6
add(["", 4]); // error

(code in playground)

---

Original answer:

You can do this:

const add: (nums: number[]) => number = ([x, ...xs]) => {
    if (x === undefined)
        return 0
    else
        return x + add(xs)
}

You can also use a type alias:

type AddFunction = (nums: number[]) => number;

const add: AddFunction = ([x, ...xs]) => {
    ...
}