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TypeScript · module 19 of 21

Typed APIs & Runtime Validation

Separate compile-time types from untrusted runtime data and validate API boundaries before values enter the application.

Course map
APIsends anythingvalidate hereparse, do not assumetyped datasafe from here inwardsWithout it:const u = await res.json() as User;`as` is a promise you make to the compiler, not a check. If a field is missing nothing complainsuntil something reads it — usually far away from the code that was actually wrong.
Typed APIs & Runtime Validation — the idea in one picture

What this module covers

5 steps · TypeScript

  1. Step 1. Lesson: Typed APIs & Runtime Validation

    Separate compile-time types from untrusted runtime data and validate API boundaries before values enter the application. Learning objectives - Recognize when typed apis & runtime validation is the right tool. - Explain the role of unknown, interface, validate. - Build a small working example before combining it with a larger interface. - Identify one accessibility, maintainability, or error-handling improvement. Read the example, predict what it will do, then complete the small change described in the check question.

    Separate compile-time types from untrusted runtime data and validate API boundaries before values enter the application. In TypeScript, make the state/interaction visible in the UI so completion is easy to verify.

    Check yourself: [TypeScript] Use the example to demonstrate typed apis & runtime validation.

  2. Step 2. Quiz: Typed APIs & Runtime Validation

    Check your understanding of Typed APIs & Runtime Validation.

    Type information is erased during compilation, so untrusted runtime data still needs validation. In TypeScript, make the state/interaction visible in the UI so completion is easy to verify.

    Check yourself: [TypeScript] Complete the knowledge check.

  3. Step 3. Exercise: Typed APIs & Runtime Validation

    Validate an unknown API payload and return a correctly narrowed domain type.

    This exercise turns the Typed APIs & Runtime Validation lesson into a working code change. In TypeScript, make the state/interaction visible in the UI so completion is easy to verify.

    Check yourself: [TypeScript] Does the solution use unknown and interface and validate?

  4. Step 4. Review lab: Explain & improve Typed APIs & Runtime Validation

    Rebuild the exercise without copying the solution. Then improve it in two ways: make the code easier to understand and add one useful edge case, responsive behavior, accessibility improvement, or error state. Finally, write a short explanation of how unknown, interface, validate work together.

    Rebuilding from memory and improving an edge case turns recognition into durable skill. In TypeScript, make the state/interaction visible in the UI so completion is easy to verify.

    Check yourself: [TypeScript] Does the improved version still use unknown, interface, validate clearly?

  5. Step 5. Mini project: Typed APIs & Runtime Validation

    Build a typed API client with request types, runtime response validation, normalized errors, and pagination metadata.

    The mini project combines the module lesson, quiz, and exercise into one portfolio-ready result. In TypeScript, make the state/interaction visible in the UI so completion is easy to verify.

    Check yourself: [TypeScript] Does the project apply the module's typed apis & runtime validation skills?

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