TypeScript Advanced Test

Challenge your knowledge of generics, utility types, narrowing, keyof and exhaustive checks.

6 questions. Choose the best answer for each question, then review every explanation.

Question 1 of 6

1 of 6

Question 1What does the type parameter preserve in this function?

function identity<T>(value: T): T { return value; }

A fixed string return typeA global variable named TThe relationship between the input and output typesOnly the runtime constructor name

Question 2What does keyof User produce?

interface User { id: number; name: string }

"id" | "name"number | stringUser[]keyof

Question 3What does Partial<User> do?

Makes every property readonlyAllows only one propertyMakes every User property optionalRemoves every User property

Question 4Which check narrows value from unknown to string?

typeof value === "string"value as stringBoolean(value)value !== undefined

Question 5What does the never type represent?

A property that is optionalA value that cannot occurAny nullable valueAn unresolved Promise

Question 6What does readonly enforce on a property?

interface Item { readonly id: number }

JavaScript freezes the object at runtimeThe property is hidden from loopsThe property can only contain stringsTypeScript rejects reassignment through that property