Readonly class properties in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the <code>readonly</code> keyword do in a TypeScript class property?

It makes the property value unchangeable after it is initialized, preventing any reassignment.

beginner

Can a readonly property be assigned a value inside the constructor?

Yes, readonly properties can be assigned or modified inside the constructor but not outside of it.

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What happens if you try to assign a new value to a readonly property outside the constructor?

TypeScript will show a compile-time error preventing the reassignment.

beginner

Example: <pre>class Car { readonly model: string; constructor(model: string) { this.model = model; } }</pre> Can you change <code>model</code> after creating a <code>Car</code> object?

No, once the model is set in the constructor, it cannot be changed later because it is readonly.

beginner

Why use readonly properties in classes?

To protect important data from accidental changes and make the code safer and easier to understand.

What is true about a readonly property in TypeScript?

A It can only be assigned once, typically in the constructor.

B It can be changed anytime after object creation.

C It is the same as a private property.

D It can only hold numbers.

A readonly property can be assigned once, usually in the constructor, and cannot be changed later.

Where can you assign a value to a readonly property?

A Only outside the class.

B Anywhere in the class methods.

C Inside the constructor.

D In any function.

You can assign or modify a readonly property inside the constructor.

What error occurs if you try to change a readonly property after construction?

A Runtime error.

B Compile-time error.

C No error, it changes normally.

D Syntax error.

TypeScript prevents reassignment of readonly properties with a compile-time error.

Which keyword makes a class property immutable after initialization?

A readonly

B const

C private

D static

readonly makes a property immutable after it is initialized.

Why might you choose readonly over private?

A <code>private</code> makes the property constant.

B <code>private</code> allows changes from outside.

C <code>readonly</code> hides the property completely.

D <code>readonly</code> allows read access but prevents changes.

readonly lets others read the property but stops them from changing it, unlike private which restricts access.

Explain how readonly properties work in TypeScript classes and when you can assign their values.

Think about when and where you can set the value and what happens if you try to change it later.

Describe a real-life example where using readonly properties in a class would be helpful.

Imagine something that should never change once set, like a serial number or birthdate.