How to create a circularly referenced type in TypeScript?

Question

I have the following code:

type Document = number | string | Array;

TypeScript complains with the following error:

Answer 1

Here is one way to do it:

class Doc {
  val: number | string | Doc[];
}

let doc1: Doc = { val: 42 };
let doc2: Doc = { val: "the answer" };
let doc3: Doc = { val: [doc1, doc2] };

Types that reference themselves are known as "recursive types" and are discussed in section 3.11.8 of the language spec. The following excerpt explains why your attempt does not compile:

Classes and interfaces can reference themselves in their internal structure...

Your original example uses neither a class nor an interface; it uses a type alias.

Answer 2

We already have good answers, but I think we can get closer to what you wanted in the first place:

You may try something like this:

interface Document {
    [index: number]: number | string | Document;
}

// compiles
const doc1: Document = [1, "one", [2, "two", [3, "three"]]];

// fails with "Index signatures are incompatible" which probably is what you want
const doc2: Document = [1, "one", [2, "two", { "three": 3 }]];

Compared to NPE's answer, you don't need wrapper objects around strings and numbers.

If you want a single number or string to be a valid document (which is not what you asked, but what NPE's answer implies), you may try this:

type ScalarDocument = number | string;
interface DocumentArray {
    [index: number]: ScalarDocument | DocumentArray;
}
type Document = ScalarDocument | DocumentArray;

const doc1: Document = 1;
const doc2: Document = "one";
const doc3: Document = [ doc1, doc2 ];

Update:

Using an interface with index signature instead of an array has the disadvantage of losing type information. Typescript won't let you call array methods like find, map or forEach. Example:

type ScalarDocument = number | string;
interface DocumentArray {
    [index: number]: ScalarDocument | DocumentArray;
}
type Document = ScalarDocument | DocumentArray;

const doc1: Document = 1;
const doc2: Document = "one";
const doc3: Document = [ doc1, doc2 ];
const doc = Math.random() < 0.5 ? doc1 : (Math.random() < 0.5 ? doc2 : doc3);

if (typeof doc === "number") {
    doc - 1;
} else if (typeof doc === "string") {
    doc.toUpperCase();
} else {
    // fails with "Property 'map' does not exist on type 'DocumentArray'"
    doc.map(d => d);
}

This can be solved by changing the definition of DocumentArray:

interface DocumentArray extends Array<ScalarDocument | DocumentArray> {}

Answer 3

Building on what NPE said, types cannot recursively point to themselves, you could unroll this type to whatever level of depth you considered sufficient, e.g.:

type Document = [number|string|[number|string|[number|string|[number|string]]]]

Not pretty, but removes the need for an interface or class with a property value.

Answer 4

The creator of TypeScript explains how to create recursive types here.

The workaround for the circular reference is to use extends Array. In your case this would lead to this solution:

type Document = number | string | DocumentArray;

interface DocumentArray extends Array<Document> { }

Update (TypeScript 3.7)

Starting with TypeScript 3.7, recursive type aliases will be permitted and the workaround will no longer be needed. See: https://github.com/microsoft/TypeScript/pull/33050