Conditional Types + infer (Interview Essentials)

30‑second interview answer

Conditional types let TypeScript compute a type based on a condition (T extends U ? X : Y). infer lets you capture part of a type inside a conditional (like extracting a promise’s resolved type). They power many utility types and advanced generic APIs.

Conditional types

type IsString<T> = T extends string ? true : false;
 
type A = IsString<'x'>; // true
type B = IsString<123>; // false

infer: extract inner types

Extract array element

type Element<T> = T extends (infer U)[] ? U : never;
 
type X = Element<string[]>; // string

Extract promise resolved type

type AwaitedLike<T> = T extends Promise<infer U> ? U : T;
 
type R1 = AwaitedLike<Promise<number>>; // number
type R2 = AwaitedLike<string>; // string

Real-world use: function return type

type Return<T> = T extends (...args: any[]) => infer R ? R : never;

Common pitfalls

• Distributive conditional types on unions
• Getting never unexpectedly

Interview-safe line:

“These are powerful, but I keep them small and tested. I prefer clarity over clever.”

Mini Q&A

Q1: What is a conditional type?

• A type-level if/else.

Q2: What does infer do?

• Captures a type variable from inside another type.

Q3: Where are these used?

• Utility types, libraries like React Query, typed APIs.

Summary checklist

• I can write a conditional type.
• I can use infer to extract inner types.
• I understand union distribution basics.