Question 1

What problem do generics solve?

Accepted Answer

Parametrize types for reusable, type-safe APIs without losing specificity. identity<T>(x: T): T preserves input type vs any. Constraints limit T.

Question 2

Explain generic constraints (extends).

Accepted Answer

Restrict type parameters so you can access known members safely. T extends { id: string } enables x.id. Constraints compose carefully with defaults.

Question 3

Default type parameters — when useful?

Accepted Answer

Provide sensible defaults so callers needn't always specify args. Common in component props and builder APIs. Defaults still constrained.

Question 4

keyof and indexed access with generics?

Accepted Answer

keyof T gets keys; T[K] gets value types — foundation of type-safe pickers. K extends keyof T is the classic pattern. Remapping in mapped types builds on this.

Question 5

Generic variance intuition (in/out)?

Accepted Answer

TypeScript uses bivariant hacks historically for methods; explicit in/out variance annotations exist for type params. Important for complex library APIs. Prefer immutable patterns to avoid pitfalls.

Question 6

Why inference sometimes fails with generics?

Accepted Answer

Too few contextual clues, multiple candidates, or unconstrained excess. Add contextual types, defaults, or helper functions. Avoid over-complex signatures.

Question 7

Generics vs overloads — how to choose?

Accepted Answer

Generics for parametric polymorphism; overloads for discrete input/output pairings. Overloads can be clearer for heterogeneous cases; keep list ordered from specific to general.

Question 8

Design a type-safe event emitter with generics.

Accepted Answer

Map event names to payload types; on/emit constrained by keyof map. Prevents wrong payload types. Consider once/off and async handlers.

Generics TypeScript Interview Questions

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