What are recursive types? | Interview Question

Q: What are recursive types?

**Target Audience:** Developers who know unions, object types and generics, and need to describe nested data or compute types by recursion. **Difficulty:** Hard **TypeScript version:** Every type claim and depth measurement below was checked by TypeScript 5.4.5 with `--strict`; release facts (3.7, 4.1, 4.5) are cited, not run. > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Type claims are checked by `tsc` (an `Equal ` line or a `// @ts-expect-error` line), and the **Verified** section quotes the real output in full. ## 1. Why This Even Matters — A Story First How do you describe a folder on your computer? "A folder has a name, some files, and **some folders**." The definition uses the word it is defining — and that is fine, because each inner folder is smaller, and eventually you reach folders with no folders inside. Nobody needs to know in advance how de

The problem

Question presented to candidate: "What are recursive types in TypeScript? Give me a data structure you would model with one, and then a type-level computation that uses recursion. What limits do you run into?"

What a strong answer should cover:

A recursive type refers to itself, to describe nesting of unknown depth: Json, trees, linked lists, expression ASTs, including mutual recursion between two types
The self-reference must go through a structure (object, array, tuple, generic); a bare type Loop = Loop | string is error TS2456. Type aliases became much more flexible here in TypeScript 3.7
Recursive functions walk recursive types, and a discriminated union makes every case checked
Recursive conditional types (TypeScript 4.1) compute types: deep array elements, all dotted paths of an object, tuple-length arithmetic
The depth limit (error TS2589) and tail-recursive conditional types (TypeScript 4.5), which go about a thousand levels instead of a few dozen; the accumulator pattern

Code / implementation expected: Yes — a Json or tree type with a function over it, and one recursive conditional type.

recursive-types

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Developers who know unions, object types and generics, and need to describe nested data or compute types by recursion. Difficulty: Hard TypeScript version: Every type claim and depth measurement below was checked by TypeScript 5.4.5 with --strict; release facts (3.7, 4.1, 4.5) are cited, not run.

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Type claims are checked by tsc (an Equal line or a // @ts-expect-error line), and the Verified section quotes the real output in full.

1. Why This Even Matters — A Story First

How do you describe a folder on your computer? "A folder has a name, some files, and some folders." The definition uses the word it is defining — and that is fine, because each inner folder is smaller,

The solution is hidden so you can try first

11 min read · 9 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

TypeScriptRecursive data types: Json, trees, lists and mutual recursion

Practice

TypeScriptRecursive type-level computation and depth limits

Practice

Discussion

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