Readonly properties in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the readonly keyword do in TypeScript?

It makes a property unchangeable after it is initialized. You can set it once, but cannot modify it later.

beginner

How do you declare a readonly property in a TypeScript interface?

Use the readonly keyword before the property name, like readonly name: string;.

intermediate

Can you assign a value to a readonly property inside a class constructor?

Yes, you can assign a readonly property inside the constructor, but not modify it outside.

beginner

What happens if you try to modify a readonly property after initialization?

TypeScript will give a compile-time error saying you cannot assign to a readonly property.

intermediate

Can readonly properties be used with arrays or objects to make them immutable?

Readonly properties prevent reassigning the property itself, but do not make the contents of arrays or objects immutable.

What does the readonly keyword do in TypeScript?

A Prevents modification of a property after initialization

B Makes a property optional

C Allows a property to be changed anytime

D Deletes a property from an object

The readonly keyword ensures a property cannot be changed after it is first set.

Where can you assign a value to a readonly property in a class?

A Only inside the constructor

B Anywhere in the class

C Only outside the class

D Nowhere, it must be undefined

Readonly properties can be assigned or changed inside the constructor but not elsewhere.

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

A Syntax error

B Runtime error

C Compile-time error

D No error, it changes normally

TypeScript gives a compile-time error to prevent changing readonly properties.

How do you declare a readonly property in an interface?

A Use <code>const</code> before the property name

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

C Use <code>private</code> before the property name

D Use <code>static</code> before the property name

The readonly keyword is used before the property name in interfaces.

Does readonly make the contents of an array property immutable?

A Yes, but only for string arrays

B Yes, it makes the array contents immutable

C No, it deletes the array contents

D No, it only prevents reassigning the property itself

Readonly prevents changing the property reference, but array elements can still be changed.

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

Think about when and where you can set a readonly property.

Describe the difference between readonly properties and immutability of objects or arrays in TypeScript.

Consider what readonly protects and what it does not.