Interface declaration syntax in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an interface in TypeScript?

An interface in TypeScript defines a structure for objects, specifying what properties and methods they should have without implementing them.

beginner

How do you declare a simple interface with two properties: name (string) and age (number)?

Use the interface keyword followed by the interface name and property types:<br>

interface Person {
  name: string;
  age: number;
}

intermediate

Can interfaces in TypeScript include methods? How do you declare one?

Yes, interfaces can include method signatures without implementation. For example:<br>

interface Person {
  greet(): void;
}

This means any object implementing Person must have a greet method that returns nothing.

intermediate

What does it mean when a property in an interface is marked optional with a question mark (?)?

An optional property means objects implementing the interface may or may not have that property. For example:<br>

interface Person {
  middleName?: string;
}

The middleName property can be missing.

intermediate

How do you extend an interface from another interface in TypeScript?

Use the extends keyword to create a new interface that includes all properties of the original plus new ones:<br>

interface Employee extends Person {
  employeeId: number;
}

Which keyword is used to declare an interface in TypeScript?

A class

B interface

C type

D struct

The interface keyword is used to declare interfaces in TypeScript.

How do you mark a property as optional in an interface?

A Use the keyword optional before the property

B Set the property type to undefined

C Add a question mark (?) after the property name

D Use the keyword nullable

A question mark (?) after the property name marks it as optional.

What does this interface declaration mean?<br>

interface Car {<br>  drive(): void;<br>}

A drive is a method that must return a string

B drive is a property of type void

C drive is a variable to store a function

D Any object with a drive method that returns nothing matches this interface

The interface requires a drive method that returns nothing (void).

How do you create an interface that includes all properties of another interface plus new ones?

A Use the extends keyword

B Use the implements keyword

C Use the include keyword

D Use the inherit keyword

The extends keyword is used to inherit properties from another interface.

Which of these is a valid interface declaration?

A interface User { id: number; name: string; }

B interface User = { id: number; name: string; }

C interface User (id: number, name: string)

D interface User { id => number; name => string; }

Option A uses correct TypeScript interface syntax.

Explain how to declare an interface with optional properties and methods in TypeScript.

Think about how to mark properties optional and how methods are declared without code.

Describe how interface inheritance works in TypeScript and why it is useful.

Consider how one interface can build on another.