How would you build a DeepPartial type? | Interview Question

Q: How would you build a DeepPartial type?

**Target Audience:** Developers who know `Partial`, mapped types and conditional types, and need optional-at-every-level types for config, patches or test data. **Difficulty:** Hard **TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`. > **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 A new app ships with factory settings: server on `localhost:8080`, TLS off, three retries, the `log` plugin. In production you only want to change two things — the port and TLS. Writing out the whole settings object again is tedious and error-prone. What you want to say is "everything as default, **except** `server.port` is 443 and `server.tls.enable

The problem

Question presented to candidate: "Partial<T> only makes the top level optional. How would you build a DeepPartial<T> that makes every nested property optional — and what would you watch out for?"

What a strong answer should cover:

The basic idea: a mapped type with ? that recurses into object-typed properties
Why the one-liner T[K] extends object ? DeepPartial<T[K]> : T[K] breaks: X | null is not distributed so it is not recursed into, and Dates and functions turn into bags of optional members ({} is accepted, functions stop being callable)
A robust version: leaf types (primitives, Date, RegExp, functions) pass through, a naked T distributes over unions, arrays and tuples keep their shape, Map and Set handled explicitly
Typical use: config overrides and test fixtures, merged into defaults by a deep-merge function whose behaviour matches the type (arrays replace, undefined keeps the default)
Caveats: explicit undefined is allowed under plain strict (exactOptionalPropertyTypes changes that), and inside generic code DeepPartial<T> stays unresolved

Clarifying questions expected:

Should arrays become partial element by element, or be replaced as a whole?
Which types are leaves for this codebase (dates, class instances, branded types)?

Code / implementation expected: Yes — write DeepPartial<T>, show where the naive version fails, and use it in a typed merge.

partial-deep

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 Partial, mapped types and conditional types, and need optional-at-every-level types for config, patches or test data. Difficulty: Hard TypeScript version: Every type claim below was checked by TypeScript 5.4.5 with --strict.

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

A new app ships with factory settings: server on localhost:8080, TLS off, three retries, the log plugin. In production you only want to change two things — the port and TLS. Writing out the whole settings object again is tedious and error-prone. What yo

The solution is hidden so you can try first

11 min read · 10 sections

Run the code

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

TypeScriptThe naive DeepPartial, its three bugs and a robust version

Practice

TypeScriptTyped config overrides with a matching deepMerge

Practice

Discussion

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