Why advanced utility types matter in Typescript - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a type that makes all properties optional.

Typescript

type PartialUser = [1]<User>;

Drag options to blanks, or click blank then click option'

APick

BRequired

CReadonly

DPartial

Attempts:

3 left

💡 Hint

Common Mistakes

Using Required instead of Partial, which makes all properties required.

Using Readonly which only makes properties immutable.

The Partial utility type makes all properties of a type optional.

2fill in blank

medium

Complete the code to create a type that picks only the 'name' and 'age' properties.

Typescript

type NameAndAge = [1]<User, 'name' | 'age'>;

Drag options to blanks, or click blank then click option'

APartial

BOmit

CPick

DRecord

Attempts:

3 left

💡 Hint

Common Mistakes

Using Omit which excludes properties instead of selecting them.

Using Record which creates a new type from keys and values.

The Pick utility type selects specific properties from a type.

3fill in blank

hard

Fix the error in the code to make all properties readonly.

Typescript

type ReadonlyUser = [1]<User>;

Drag options to blanks, or click blank then click option'

AReadonly

BRequired

CPartial

DPick

Attempts:

3 left

💡 Hint

Common Mistakes

Using Partial which makes properties optional but not readonly.

Using Required which makes properties required but not readonly.

The Readonly utility type makes all properties immutable.

4fill in blank

hard

Fill both blanks to create a type that excludes 'password' and makes the rest required.

Typescript

type SafeUser = [1]<User> & [2]<User, 'password'>;

Drag options to blanks, or click blank then click option'

AOmit

BPartial

CRequired

DPick

Attempts:

3 left

💡 Hint

Common Mistakes

Using Partial instead of Required, which makes properties optional.

Using Pick instead of Omit, which selects properties instead of excluding.

Omit removes the 'password' property, and Required makes the rest required.

5fill in blank

hard

Fill all three blanks to create a type that maps the keys 'active' and 'admin' to boolean and makes them readonly.

Typescript

type Flags = Readonly<[1]<[2], [3]>>;

Drag options to blanks, or click blank then click option'

ARecord

B'active' | 'admin'

Cboolean

DPick

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the order of utility types.

Using string instead of boolean for the mapped type.

Record creates a type with keys mapped to boolean values, and Readonly makes them immutable.

Practice

1. What is the main benefit of using advanced utility types in TypeScript?

easy

A. They replace the need for interfaces entirely.

B. They make your code run faster at runtime.

C. They automatically generate UI components.

D. They allow you to modify existing types without rewriting them.

Solution

1. Step 1: Understand the purpose of utility types

   Utility types help transform or modify existing types without rewriting the whole type from scratch.

2. Step 2: Compare options to the purpose

   Only They allow you to modify existing types without rewriting them. correctly states this benefit. Options B, C, and D describe unrelated features.

3. Final Answer:

   They allow you to modify existing types without rewriting them. -> Option D

4. Quick Check:

   Advanced utility types = modify types easily [OK]

Hint: Utility types change types without rewriting [OK]

Common Mistakes:

• Thinking utility types affect runtime speed
• Confusing utility types with UI generation
• Believing utility types replace interfaces

2. Which of the following is the correct syntax to make all properties of a type optional using a utility type?

easy

A. type PartialType = Readonly<MyType>;

B. type PartialType = Required<MyType>;

C. type PartialType = Partial<MyType>;

D. type PartialType = Pick<MyType>;

Solution

1. Step 1: Recall utility types for property modifiers

   Partial<T> makes all properties optional, Required<T> makes all required, Readonly<T> makes all readonly, Pick<T> selects properties.

2. Step 2: Match syntax to making properties optional

   Partial<MyType> is the correct syntax to make all properties optional.

3. Final Answer:

   type PartialType = Partial<MyType>; -> Option C

4. Quick Check:

   Partial = optional properties [OK]

Hint: Partial makes all properties optional [OK]

Common Mistakes:

• Using Required instead of Partial
• Confusing Readonly with Partial
• Using Pick without specifying keys

3. What will be the output type of this code?

type User = { id: number; name: string; age?: number };
type RequiredUser = Required<User>;

medium

A. { id: number; name: string; age?: number }

B. { id: number; name: string; age: number }

C. { id?: number; name?: string; age?: number }

D. Error: Cannot use Required on User

Solution

1. Step 1: Understand Required utility type

   Required<T> makes all properties required, removing optional modifiers.

2. Step 2: Apply Required to User type

   Original age property was optional (age?), after Required it becomes required (age: number).

3. Final Answer:

   { id: number; name: string; age: number } -> Option B

4. Quick Check:

   Required removes optional marks [OK]

Hint: Required makes all properties mandatory [OK]

Common Mistakes:

• Thinking Required keeps optional properties
• Confusing Required with Partial
• Expecting a runtime error

4. Identify the error in this TypeScript code using utility types:

type Person = { name: string; age: number };
type ReadonlyPerson = Readonly<Person>;
const readonlyPerson: ReadonlyPerson = { name: "Bob", age: 30 };
readonlyPerson.name = "Alice";

medium

A. Cannot assign to 'name' because it is a read-only property.

B. Missing semicolon after type declaration.

C. Readonly utility type is not imported.

D. Person type must have optional properties.

Solution

1. Step 1: Understand Readonly utility type effect

   Readonly<T> makes all properties read-only, so they cannot be reassigned.

2. Step 2: Analyze the assignment to readonlyPerson.name

   Trying to assign "Alice" to a read-only property causes a TypeScript error.

3. Final Answer:

   Cannot assign to 'name' because it is a read-only property. -> Option A

4. Quick Check:

   Readonly properties cannot be reassigned [OK]

Hint: Readonly properties cannot be changed [OK]

Common Mistakes:

• Ignoring read-only restrictions
• Assuming utility types need imports
• Thinking optional properties are required

5. Given this nested type, how can you create a new type that makes the properties of the 'contact' object optional while keeping 'username' required?

type Profile = {
  username: string;
  contact: {
    email: string;
    phone: string;
  };
};

hard

A. type NewProfile = { username: string; contact: Partial<{ email: string; phone: string; }> };

B. type NewProfile = Partial<Profile>;

C. type NewProfile = Required<Profile>;

D. type NewProfile = { username?: string; contact?: { email?: string; phone?: string } };

Solution

1. Step 1: Identify the target properties to make optional

   The properties inside 'contact' should be optional; 'username' remains required.

2. Step 2: Use Partial on the nested 'contact' object

   Applying Partial to 'contact' makes all its properties optional, so we use Partial only on 'contact' type, keeping 'username' required.

3. Final Answer:

   type NewProfile = { username: string; contact: Partial<{ email: string; phone: string; }> }; -> Option A

4. Quick Check:

   Partial on nested object = optional nested props [OK]

Hint: Use Partial only on nested object to make some props optional [OK]

Common Mistakes:

• Making entire Profile partial instead of nested
• Using Required which makes all required
• Making all top-level properties optional