Readonly properties in interfaces in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the readonly keyword do in a TypeScript interface?

It makes a property unchangeable after the object is created. You can read the value but cannot modify it.

beginner

How do you declare a readonly property name of type string in an interface?

Use readonly name: string; inside the interface definition.

beginner

Can you assign a new value to a readonly property after the object is created?

No, TypeScript will give an error if you try to change a readonly property after initialization.

intermediate

Why use readonly properties in interfaces? Give a real-life example.

To protect important data from accidental changes. For example, a user ID should not change once set.

beginner

Can readonly properties be set during object creation?

Yes, readonly properties can be set when the object is first created but not changed later.

What happens if you try to change a readonly property in TypeScript?

A TypeScript shows an error.

B The property changes without error.

C The program crashes at runtime.

D The property becomes undefined.

Readonly properties cannot be changed after initialization, so TypeScript shows a compile-time error.

How do you mark a property as readonly in a TypeScript interface?

A Use the keyword <code>final</code> before the property name.

B Use the keyword <code>readonly</code> before the property name.

C Use the keyword <code>immutable</code> after the property name.

D Use the keyword <code>const</code> before the property name.

The readonly keyword is used before the property name to make it unchangeable.

Can a readonly property be assigned a value when creating an object?

A Yes, but only if the property is optional.

B No, it must be set later.

C No, it cannot be set at all.

D Yes, it can be set during object creation.

Readonly properties can be assigned a value when the object is created but cannot be changed afterward.

Which of these is a good reason to use readonly properties?

A To prevent accidental changes to important data.

B To make properties private.

C To allow properties to be changed anytime.

D To improve runtime performance.

Readonly properties help protect important data from being changed by mistake.

If an interface has a readonly property, can you omit it when creating an object?

A Yes, readonly properties are always optional.

B No, readonly means it must be included.

C Only if the property is optional.

D Yes, readonly properties are ignored.

Readonly does not mean required; if the property is optional, you can omit it.

Explain what readonly properties in interfaces are and why they are useful.

Think about how you keep some information fixed, like a birthdate.

Describe how to declare and use a readonly property in a TypeScript interface with a simple example.

Show how to write the interface and create an object with that property.