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

Check Yourself: Generics Without the Fog

Five scenario questions on type parameters, constraints and generic types. 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. A helper is declared as function firstOf(items: any[]): any and returns items[0]. You call it with [19.5, 20, 7] and then call toUpperCase on the result. What does the compiler do, and what is the better declaration?
It accepts the call, and nothing can be done about it, because arrays always give back any. It rejects the call, because any is not allowed in strict mode. It accepts the call, because the result is any. A type parameter, written firstOf with T, items as an array of T and a return type of T or undefined, keeps the number type so the call is rejected. It rejects the call, because numbers have no toUpperCase method.
2. You write function longest with T extending an object type with a numeric length property, taking two parameters of type T and returning T. A teammate calls longest(10, 20). What happens?
The call compiles and returns 20, because numbers can be compared. The call compiles, and length is undefined at run time. The compiler reports an error, because a number has no length property and so does not satisfy the constraint. The compiler changes T to any so that the call works.
3. mapAll takes items as an array of T and a callback from T to U, and returns an array of U. You call mapAll with ["a", "bb", "ccc"] and a callback that returns s.length. What is the type of the result?
An array of string, because the input items are strings. It cannot be known until the code runs. An array of any, because the callback has no annotation. An array of number, because U comes from what the callback returns.
4. You declare type Page with a type parameter T, holding an array of T and a total, and then write const users: Page = { items: [], total: 0 }. What does the compiler say?
That Page requires one type argument, so you write something like Page of User. That the total property is not allowed. Nothing, because the empty array makes T any. Nothing, because T defaults to string.
5. A teammate writes function logValue with a type parameter T, one parameter of type T, and a void return. You are asked to review it. What is the most useful comment?
It will not compile, because a generic function must return T. T appears in only one place, so it links nothing to anything else. A plain unknown parameter says the same thing with less syntax. It is fine, because every function should be generic where possible. It should use any instead of T, so that it accepts every value.

About

Five scenario questions on type parameters, constraints and generic types. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/12/2026
Was this helpful?
​ 0 ​ 0

Comments (0)

Sign in to comment

No comments yet.