Recursive array type typescript

Issue

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?

Solution

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 =  ""

Answered By - Titian Cernicova-Dragomir