Question 1

What is a user-defined type guard?

Accepted Answer

Function returning arg is Type that narrows when true. Implement runtime check correctly — TS trusts the predicate.

Question 2

instanceof vs custom guards?

Accepted Answer

instanceof uses prototype chain; custom guards for interfaces/plain objects. Interfaces don't exist at runtime — need discriminants or validators.

Question 3

How do you type-guard arrays of unknowns?

Accepted Answer

Array.isArray plus element predicates; consider every() for homogeneous arrays. For tuples, length and positional checks. Validate externally sourced JSON carefully.

Question 4

Zod/io-ts vs hand-written guards?

Accepted Answer

Libraries give parsers + inferred types; hand guards are lighter but error-prone. Prefer schema libs at trust boundaries (API, forms).

Question 5

Exhaustiveness checks with never?

Accepted Answer

In default/switch, assign residual to never to force handling new variants. Compile-time safety for evolving unions. Pair with discriminants.

Question 6

Type predicates with generics?

Accepted Answer

Guards can be generic to narrow to T when runtime check confirms. Keep predicates honest. Avoid x is T without testing T's shape.

Question 7

Security note on type guards?

Accepted Answer

Guards are compile-time illusions unless runtime validation is correct. Never trust casts. Validate untrusted input at boundaries.

Type Guards TypeScript Interview Questions

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