Why advanced utility types matter in Typescript - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code using utility types?

Consider this TypeScript code using Partial and Required utility types. What will be the output when running the code?

Typescript

type Person = {
  name: string;
  age?: number;
};

const partialPerson: Partial<Person> = { name: "Alice" };
const requiredPerson: Required<Person> = { name: "Bob", age: 30 };

console.log(partialPerson.age);
console.log(requiredPerson.age);

A 30\n30

B undefined\n30

C undefined\nundefined

D Error: Property 'age' is missing in type '{ name: string; }' but required in type 'Required<Person>'

💡 Hint

Think about what Partial and Required do to the properties of a type.

Partial makes all properties optional, so age can be missing and is undefined. Required makes all properties required, so age must be present and is 30.

🧠 Conceptual

intermediate

Why use the Pick utility type?

Which of the following best explains why the Pick utility type is useful in TypeScript?

A It converts all properties of a type to readonly.

B It removes all optional properties from a type.

C It creates a new type by selecting specific properties from an existing type.

D It merges two types into one.

💡 Hint

Think about how you can create smaller types from bigger ones.

Pick lets you create a new type by choosing only some properties from another type, which helps keep code focused and safe.

🔧 Debug

advanced

What error does this TypeScript code produce?

Look at this code using Record utility type. What error will TypeScript show?

Typescript

type Roles = 'admin' | 'user';

const rolePermissions: Record<Roles, string[]> = {
  admin: ['read', 'write'],
  user: ['read'],
  guest: ['read']
};

A Error: Object literal may only specify known properties, and 'guest' does not exist in type 'Record<Roles, string[]>'

B No error, code compiles fine.

C Error: Missing property 'user' in object.

D Error: Type 'string[]' is not assignable to type 'string'.

💡 Hint

Check if all keys in the object match the keys in the Record type.

Record requires exactly the keys specified. 'guest' is not part of 'Roles', so TypeScript errors.

📝 Syntax

advanced

Which option correctly uses the Exclude utility type?

Given type T = 'a' | 'b' | 'c', which option correctly excludes 'b' from T?

A type U = Exclude<'b', T>; // 'a' | 'c'

B type U = Exclude<T, 'd'>; // 'a' | 'b' | 'c'

C type U = Exclude<T, 'a' | 'c'>; // 'b'

D type U = Exclude<T, 'b'>; // 'a' | 'c'

💡 Hint

Remember the order of parameters in Exclude: first is the union, second is what to remove.

Exclude removes 'b' from T, leaving 'a' | 'c'. Other options either reverse parameters or exclude wrong values.

🚀 Application

expert

How many properties does this mapped type have?

Given this code, how many properties does the type Flags have?

Typescript

type Keys = 'featureA' | 'featureB' | 'featureC';

type Flags = {
  [K in Keys as K extends 'featureB' ? never : K]: boolean;
};

A 2

B 3

C 1

D 0

💡 Hint

Look at the conditional key remapping in the mapped type.

The mapped type excludes 'featureB' by mapping it to never, so only 'featureA' and 'featureC' remain, totaling 2 properties.