Recursive array type typescript

Question

Say I have the type type Atom = string | boolean | number. I want to define a type of array like:

NestedArray = Atom | [a_0, a_1, ... , a_n] where each a_i is an Atom, or a NestedArray.

Can this be achieved in Typescript?

Answer 1

Type aliases can't reference themselves, so this naïve approach will fail:

type NestedArray = Atom | Array<NestedArray | Atom> //Type alias 'NestedArray' circularly references itself.

Interfaces can however reference themselves:

interface NestedArray extends Array<NestedArray | Atom> {

}

And we can define an extra union at the top level to handle the root case:

type Atom = string | boolean | number

interface NestedArray extends Array<NestedArray | Atom> {

}

type AtomOrArray = Atom | NestedArray;

//Usage
let foo: AtomOrArray = [
    "",
    1, 
    [1, 2, ""]
]   

let bar: AtomOrArray =  ""

Answer 2

Now typescript allow type circularly references itself, such like this:

type RArray = (string | RArray)[];

And if you want to define a generic type in recursive function, can use interface ...<T> extends (T|...<T>)[]

interface RA<T> extends Array<T | RA<T>> { }

For example, create a function to get sum recursivily

function sum(arr: RA<number>) {
  let res = 0;
  arr.forEach((n) => {
    if (Array.isArray(n)) {
      res += sum(n);
    } else {
      res += n;
    }
  });
  return res;
}
console.log(sum([1, 2, 3, [4, [5]], [[6]]]))
// output: 21

But this way has some disadvantage, compiler can't know the specific type of n

arr.forEach((n) => {
    if (Array.isArray(n)) { // typeof n: `number | RA<number>`
      res += sum(n);
    } else {
      res += n;
    }
  });