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 trying

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

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

But I have no idea how to add TypeScript types to this. My best guess is to do something like this (most direct translation)

const add = (xs: number[]): number => {
  if (xs[0] === undefined)
    return 0;
  else
    return xs[0] + add(xs.slice(1));
};

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

Both functions work, but in TypeScript I lose the ability to destructure the array parameter and the function body is junked up with a bunch of ugly stuff like xs[0] and xs.slice(1) – even if I abstract these into their own functions, that's besides the point.

Is it possible in add types to destructured spread parameters in TypeScript?

---

What I've tried so far

Something like this works for fixed arrays

// compiles
const add = ([x,y,z]: [number, number, number]): number => ...

But of course I need variable length array input. I tried this, but it doesn't compile

// does not compile
const add = ([x, ...xs]: [number, number[]]): number => ...

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]) => {
    ...
}