Extending interfaces in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to extend an interface in TypeScript?

Extending an interface means creating a new interface that inherits properties and methods from an existing interface, allowing reuse and addition of new members.

intermediate

How do you extend multiple interfaces in TypeScript?

You list multiple interfaces separated by commas after the extends keyword, like interface NewInterface extends Interface1, Interface2 {}.

intermediate

Can an interface extend a class in TypeScript?

Yes, an interface can extend a class, inheriting its members but without implementation, allowing the interface to describe the shape of the class.

advanced

What happens if an extending interface redeclares a property from the base interface with a different type?

TypeScript will raise an error if the property types are incompatible, because the extending interface must be compatible with the base interface.

beginner

Show a simple example of extending an interface with one additional property.

Example:<br>

interface Animal {
  name: string;
}
interface Dog extends Animal {
  breed: string;
}

Which keyword is used to extend an interface in TypeScript?

A implements

B extends

C inherits

D extendsInterface

The extends keyword is used to create a new interface that inherits from another interface.

Can an interface extend more than one interface?

A No, only one interface can be extended

B Yes, but only two interfaces

C No, interfaces cannot extend other interfaces

D Yes, by listing them separated by commas

An interface can extend multiple interfaces by listing them separated by commas after the extends keyword.

What is the result if an extending interface changes the type of a property from the base interface incompatibly?

A TypeScript throws a type compatibility error

B TypeScript allows it without errors

C The base interface is changed automatically

D The property is ignored

TypeScript enforces type compatibility and will throw an error if the property types conflict.

Which of the following is a valid way to extend interfaces?

A interface A extends B implements C {}

B interface A implements B, C {}

C interface A extends B, C {}

D interface A inherits B, C {}

Interfaces extend other interfaces using the extends keyword and can list multiple interfaces separated by commas.

Can an interface extend a class in TypeScript?

A Yes, interfaces can extend classes

B No, interfaces cannot extend classes

C Only abstract classes can be extended by interfaces

D Only classes with no constructor can be extended

Interfaces can extend classes to inherit their members' types without implementation.

Explain how extending interfaces helps in organizing and reusing code in TypeScript.

Think about how you can build on existing blueprints without rewriting everything.

Describe the syntax and rules for extending multiple interfaces in TypeScript.

Focus on how to list interfaces and what TypeScript expects.