Question 1

What is a conditional type?

Accepted Answer

T extends U ? X : Y — choose types based on assignability. Foundation of many utilities (Exclude, Extract). Distributes over naked type params.

Question 2

What is distributivity in conditional types?

Accepted Answer

Naked T extends U ? X : Y applied to unions distributes over members. Wrap in [] to disable. Explains Exclude behavior on unions.

Question 3

infer keyword — what does it do?

Accepted Answer

Declare a type variable to capture from extends patterns. Example: T extends (...args: infer P) => any ? P : never. Core of ReturnType.

Question 4

Extract and Exclude implemented how?

Accepted Answer

Conditional types filtering unions by assignability to a type. Useful for refining event maps and removing nullish members.

Question 5

How do you flatten Promise types with conditionals?

Accepted Answer

Recursive Awaited-like conditionals unwrapping thenables. Handle non-promise passthrough. Avoid infinite recursion.

Question 6

Debugging monstrous conditional types?

Accepted Answer

Simplify intermediates, use helper aliases, TypeScript language service hover, type tests. Split steps. Prefer readable incremental transforms.

Question 7

Library design: expose conditionals or simpler overloads?

Accepted Answer

Prefer ergonomic overloads/docs for app devs; keep heavy conditionals internal. Public types are API — optimize for error message clarity.

Conditional Types TypeScript Interview Questions

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