Q: Does this compile?

Correct answer: Yes — Partial<User makes every property of User optional, so a subset object is valid. Partial<T maps over every property of T and marks it optional (?). Since UserUpdate = Partial<User, all of id, name, and email become optional, so an object supplying only name satisfies the type. Property declaration order doesn't matter (d).

Q: What properties does ProductPreview have?

Correct answer: Only id and name, exactly as listed in the second type argument. Pick<T, K builds a new type containing only the properties named in the union K. Here K is 'id' | 'name', so ProductPreview has exactly those two properties — price is excluded entirely, not made optional.

Q: What properties does PublicProduct have?

Correct answer: id, name, and price — every property of Product except internalCode. Omit<T, K is the inverse of Pick: it keeps every property of T EXCEPT those named in K. Here K is just 'internalCode', so PublicProduct retains id, name, and price and drops internalCode.

Q: What does Record<'math' | 'science' | 'art', number describe?

Correct answer: An object type with exactly the keys 'math', 'science', 'art', each mapped to a number. Record<K, V builds an object type whose keys are exactly the members of union K, each with value type V. So Record<'math' | 'science' | 'art', number requires an object with precisely those three keys, each holding a number — omitting one of them, or adding an unlisted key, is a type error.

Q: What does ReturnType<typeof createUser produce?

Correct answer: The type of whatever createUser returns: { name: string; age: number; createdAt: Date }. typeof createUser gets the function's TYPE (its call signature), and ReturnType<F extracts the type of what a function type F returns. So NewUser becomes the inferred return object shape { name: string; age: number; createdAt: Date } — it does not preserve the function signature itself (d), and it isn't a literal string (a).

Question 1

TypeScript ships several built-in utility types that transform an existing type into a new one. Which of these is a built-in utility type?

Accepted Answer

Correct answer: Partial<T. Partial<T is one of TypeScript's built-in utility types — it produces a new type where every property of T is optional. Optional, Nullable, and Mutable are not part of the standard library (teams sometimes define their own similarly-named helpers, but they aren't built in).

Question 2

Does this compile?

Accepted Answer

Correct answer: Yes — Partial<User makes every property of User optional, so a subset object is valid. Partial<T maps over every property of T and marks it optional (?). Since UserUpdate = Partial<User, all of id, name, and email become optional, so an object supplying only name satisfies the type. Property declaration order doesn't matter (d).

Question 3

What properties does ProductPreview have?

Accepted Answer

Correct answer: Only id and name, exactly as listed in the second type argument. Pick<T, K builds a new type containing only the properties named in the union K. Here K is 'id' | 'name', so ProductPreview has exactly those two properties — price is excluded entirely, not made optional.

Question 4

What properties does PublicProduct have?

Accepted Answer

Correct answer: id, name, and price — every property of Product except internalCode. Omit<T, K is the inverse of Pick: it keeps every property of T EXCEPT those named in K. Here K is just 'internalCode', so PublicProduct retains id, name, and price and drops internalCode.

Question 5

What does Record<'math' | 'science' | 'art', number describe?

Accepted Answer

Correct answer: An object type with exactly the keys 'math', 'science', 'art', each mapped to a number. Record<K, V builds an object type whose keys are exactly the members of union K, each with value type V. So Record<'math' | 'science' | 'art', number requires an object with precisely those three keys, each holding a number — omitting one of them, or adding an unlisted key, is a type error.

Question 6

What does ReturnType<typeof createUser produce?

Accepted Answer

Correct answer: The type of whatever createUser returns: { name: string; age: number; createdAt: Date }. typeof createUser gets the function's TYPE (its call signature), and ReturnType<F extracts the type of what a function type F returns. So NewUser becomes the inferred return object shape { name: string; age: number; createdAt: Date } — it does not preserve the function signature itself (d), and it isn't a literal string (a).

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