Readonly utility type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Readonly utility type do in TypeScript?

It makes all properties of an object type read-only, so they cannot be changed after initialization.

beginner

How do you apply the Readonly utility type to an interface Person?

You write Readonly<Person> to create a new type where all properties of Person are read-only.

beginner

Can you modify properties of an object typed with Readonly after it is created?

No, properties are read-only and TypeScript will give an error if you try to assign new values.

intermediate

Example: What is the error in this code?<br><pre>type User = { name: string };
const user: Readonly&lt;User&gt; = { name: 'Alice' };
user.name = 'Bob';</pre>

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

beginner

Why use Readonly utility type in your code?

To prevent accidental changes to objects, making code safer and easier to understand by enforcing immutability.

What does the Readonly utility type do?

A Converts properties to optional

B Makes all properties of an object read-only

C Allows properties to be changed anytime

D Removes all properties from an object

Readonly makes all properties read-only, preventing changes.

How do you declare a read-only version of a type Car?

A Readonly<Car>

B Mutable<Car>

C Partial<Car>

D Required<Car>

Readonly creates a type with all properties read-only.

What happens if you try to assign a new value to a read-only property?

A The value changes successfully

B The property becomes optional

C The property is deleted

D TypeScript shows an error

TypeScript prevents assignment to read-only properties with an error.

Which of these is a benefit of using Readonly?

A Prevents accidental data changes

B Makes code run faster

C Allows dynamic property addition

D Removes type checking

Readonly helps avoid accidental changes, improving code safety.

Can Readonly be used on nested objects automatically?

A It removes nested objects

B Yes, it applies deeply by default

C No, it only applies to the first level properties

D It converts nested objects to arrays

Readonly only makes the top-level properties read-only, nested objects remain mutable unless handled separately.

Explain what the Readonly utility type does and why it is useful.

Think about how it stops you from changing object properties after creation.

Describe how you would use Readonly with a TypeScript interface and what happens if you try to modify a property.

Consider the syntax and the error you get when changing a read-only property.