Skip to Content
TypeScript Intermediate: Types That Survive Real Projects
Intermediate
0 likes
Courses
TypeScript Intermediate: Types That Survive Real Projects

Check Yourself: Strictness That Pays Off

Five scenario questions on unknown, caught errors and discriminated unions. If you miss more than one, reread the lessons in this section.

5 min TypeScriptObject-Oriented ProgrammingAPIs and IntegrationsType Systems Intermediate
Prev Next
Fullscreen Share
Course content

Sign in to save your progress.

Course Home
1. You store the result of JSON.parse in a constant typed unknown, and then read value.retries on the next line. What does the compiler do?
It accepts the line and gives retries the type any. It accepts the line, because unknown is the same as any. It reports that JSON.parse should never be used. It reports that value is of type unknown. You have to check the value first, for example with typeof and in.
2. In a strict project you write a try statement with catch (error) and call error.message inside. The compiler refuses. What is the right fix?
Move the call outside of the catch block. Declare the variable as catch (error: any). Check the value first, for example with error instanceof Error, because the caught value is unknown. Turn off strict mode.
3. A response type has three cases tagged by a status property with the values success, error and loading. The error case has code and message. Inside an if that tests status equal to the word error, what can you read on the response?
code, message and data, because the union has all of them. code and message, because the compiler narrowed the union to the error case. Only status, because the others are optional. Nothing until you cast the response.
4. You add a fourth case to the response union, with the status empty. A switch on status ends with a default that passes the response to a function taking never. What happens to the existing code?
Nothing, because default catches the new case. The compiler adds a branch for the new case automatically. The compiler reports an error at that call, because the new case is not handled and is not never. It fails only at run time, when an empty response arrives.
5. In a union of response types, one case declares its status as a plain string instead of a literal such as the word success. What happens to narrowing?
Narrowing works better, because string is more general. The compiler reports an error on the declaration. The compiler can no longer tell the cases apart by status, so checking it does not narrow the union. Narrowing still works, but only for the other cases.

About

Five scenario questions on unknown, caught errors and discriminated unions. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/08/2026
Was this helpful?
​ 0 ​ 0

Comments (0)

Sign in to comment

No comments yet.