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TypeScript Advanced: Types That Hold Up in Production
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TypeScript Advanced: Types That Hold Up in Production

Check Yourself: Types That Compute

Five scenario questions on conditional types, infer and template literal types. If you miss more than one, reread the lessons in this section.

5 min TypeScriptTesting and DebuggingPerformance OptimizationType SystemsPackaging and Dependencies Advanced
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1. You write type IsString of T as T extends string ? "yes" : "no". What is IsString of the union string or number?
Only "yes", because string is one of the members. An error, because a union cannot be checked with extends. The union "yes" or "no", because the check runs once for each member of the union and the results are joined. Only "no", because a union is not a string.
2. What does the line type ElementOf of T as T extends (infer U)[] ? U : never give for ElementOf of string[]?
string[], because the whole input is returned when it matches. never, because infer cannot be used with arrays. string, because string[] matches the pattern and U is the type at the element position. unknown, because U has no constraint.
3. A conditional type is nested three levels deep and nobody can read it. What is the recommended fix?
Add more infer keywords so that each level has its own variable. Move it to a different file so it is out of sight. Replace the whole type with any. Name each step as its own type, or use built-ins such as ReturnType and Awaited, then test the result.
4. You build the union of event names from two unions, 10 entities and 10 actions, with a template literal type. How many string types does it contain, and what is the risk as the lists grow?
100, because the members multiply, and a few larger lists can pass the compiler's limit and give error TS2590. 10, because the compiler removes duplicates in the second list. 20, because the members of the two lists are added together. 100, and there is no limit, because types are erased at run time.
5. Which statement about a template literal type such as a route pattern starting with /users/ and ending with a number is true?
It only accepts a fixed list of strings written in advance. It checks the shape of the text at compile time, and the value is still an ordinary string at run time. It converts the number part into a number value. It checks the text again at run time and throws if the pattern does not match.

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Five scenario questions on conditional types, infer and template literal types. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/11/2026
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