What do Exclude, Extract, and NonNullable do? | Interview Question

Q: What do Exclude, Extract, and NonNullable do?

**Target Audience:** Developers who know union types and want to transform them: remove members, keep members, strip `null`. **Difficulty:** Medium **TypeScript version:** Every type claim below was checked by TypeScript 5.4.5 with `--strict`; release versions 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 A box of mixed sweets arrives: toffees, mints, chocolates and two empty wrappers. Three ways to sort it: "everything except the mints" (**Exclude**), "only the chocolates" (**Extract**), "throw away the empty wrappers" (**NonNullable**). You never open a sweet or change one — you only decide which ones stay in the box. A union type is that box. `

The problem

Question presented to candidate: "What do Exclude, Extract and NonNullable do? Can you show how Exclude is implemented, and give a case where one of them surprised you?"

What a strong answer should cover:

They filter the members of a union: Exclude<T, U> drops members assignable to U, Extract<T, U> keeps only those, NonNullable<T> drops null and undefined
Exclude and Extract are one-line distributive conditional types (T extends U ? never : T); members mapped to never vanish from the union
Practical uses: narrowing a status union, picking one variant of a discriminated union with Extract<Action, { type: "add" }>, typing a filter that removes empty values, building Omit
Limits: they work on unions, not object properties (use Omit for that); Exclude<string, "a"> is still string; U is not checked against T, so typos are silent
NonNullable<T> is T & {} since TypeScript 4.8, which lets a narrowed generic value be returned as NonNullable<T>; it is shallow

Code / implementation expected: Optional — write MyExclude by hand, or use Extract to pick a union member.

exclude
extract

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Solution

Target Audience: Developers who know union types and want to transform them: remove members, keep members, strip null. Difficulty: Medium TypeScript version: Every type claim below was checked by TypeScript 5.4.5 with --strict; release versions 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

A box of mixed sweets arrives: toffees, mints, chocolates and two empty wrappers. Three ways to sort it: "everything except the mints" (Exclude), "only the chocolates" (Extract), "throw away the empty wrappers" (NonNullable). You never open a

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9 min read · 11 sections

Run the code

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

TypeScriptFiltering string unions, action unions and nullable values

Practice

TypeScriptHow they are built, a stricter Exclude and NonNullable as T & {}

Practice

Discussion

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