What are conditional types? | Interview Question

Q: What are conditional types?

**Target Audience:** Developers who know generics and want types that change depending on their input. **Difficulty:** Hard **TypeScript version:** Every compiler message and output below was produced with TypeScript 5.4.5 (`--strict`, target ES2022). > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. All output was produced by actually running the code; the **Verified** section quotes it in full. This page covers the fundamentals. How conditional types behave on **unions** (distribution, and how to switch it off) is the subject of [Why are conditional types distributive and how do you control it with `infer`?](/interview-question/why-are-conditional-types-distributive-and-how-do-you-control-it-with-infer), and `infer` itself has [its own page](/interview-question/what-does-the-infer-keyword-do). ## 1. Why This Even Matters — A Story First A coat-check desk gives

The problem

Question presented to candidate: "What is a conditional type in TypeScript? Write one, explain what extends means inside it, and tell me where you have seen them used."

What a strong answer should cover:

The form T extends U ? X : Y: a type-level if/else that picks X when T is assignable to U, otherwise Y; added in TypeScript 2.8
extends means assignability, not equality: literals fit their wide type, objects with extra properties fit, any gives both branches, the first match in a chain wins
Deferred resolution: inside a generic function the condition is unresolved, so returning a concrete value needs a cast (or overloads)
infer to pull a type out of a pattern (T extends (infer E)[] ? E : T), and that the standard Exclude, Extract and ReturnType are conditional types (NonNullable started as one in 2.8 but is T & {} in today's lib)
When the checked type is a naked type parameter the condition distributes over unions (its own topic), and never then gives never
Trade-off against function overloads for return types that depend on arguments

Code / implementation expected: Yes — a small conditional type such as TypeName<T> or a return type that depends on an argument.

conditional-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 generics and want types that change depending on their input. Difficulty: Hard TypeScript version: Every compiler message and output below was produced with TypeScript 5.4.5 (--strict, target ES2022).

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. All output was produced by actually running the code; the Verified section quotes it in full.

This page covers the fundamentals. How conditional types behave on unions (distribution, and how to switch it off) is the subject of Why are conditional types distributive and how do you control it with infer?, and infer itself has [its own page](/interview-question/wh

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10 min read · 11 sections

Run the code

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

TypeScriptConditional types, deferral and infer

Practice

TypeScriptWhat extends means: assignability edge cases

Practice

Discussion

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