Question 1

What does strict mode enable that's important?

Accepted Answer

strictNullChecks, noImplicitAny, strictFunctionTypes, etc. — core soundness options. Turn on for new projects. Migrate incrementally for legacy.

Question 2

Why is any contagious?

Accepted Answer

Flows through assignments and disables checking downstream. Ban via eslint @typescript-eslint/no-explicit-any with controlled exceptions.

Question 3

Safe patterns for JSON.parse?

Accepted Answer

Parse unknown then validate (zod); avoid casting to a detailed interface blindly. Runtime validation is part of type safety at boundaries.

Question 4

Non-null assertion (!) risks?

Accepted Answer

Tells compiler to ignore nullish — can crash at runtime if wrong. Prefer narrowing. Reserve ! for proven invariants after checks.

Question 5

Type assertion vs type conversion?

Accepted Answer

as Type is compile-time only; conversion is runtime (Number(x)). Assertions don't change values. Double assertions via unknown are escape hatches — use sparingly.

Question 6

How do you migrate a JS codebase toward type safety?

Accepted Answer

allowJs + checkJs, incremental strictness, typed boundaries first, any budgets. Don't boil the ocean. Prioritize domain core and public APIs.

Question 7

Soundness vs productivity trade-offs?

Accepted Answer

TS is intentionally unsound in places (bivariance historical, excess property limited). Understand holes. Use lint/runtime checks to compensate critical paths.

Type Safety TypeScript Interview Questions

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