Merging classes with interfaces in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to merge a class with an interface in TypeScript?

It means adding extra type information or members to a class by declaring an interface with the same name. The class and interface combine, so the class must satisfy both its own members and the interface's members.

beginner

How do you declare an interface to merge with an existing class named <code>Person</code>?

You declare an interface with the exact same name: <br><pre>interface Person { <br>  newProperty: string; <br>}</pre> This adds <code>newProperty</code> to the <code>Person</code> class instance type.

intermediate

Can merged interfaces add properties to the class's instance type or static type?

Merged interfaces add properties to the class's instance type, not the static type. To add static members, you must merge with the class's constructor type separately.

intermediate

What happens if the class does not implement the members declared in the merged interface?

TypeScript will show a type error because the class instance does not satisfy the interface's contract. You must implement all interface members in the class or provide them via other means.

beginner

Why is merging classes with interfaces useful in TypeScript?

It allows you to extend or add type information to existing classes without modifying their original code. This is helpful for adding properties or methods for typing purposes, especially with third-party libraries.

What must an interface have to merge correctly with a class in TypeScript?

A The same name as the class

B The same properties as the class

C A different name than the class

D Only static members

Interfaces merge with classes only if they share the exact same name.

When merging an interface with a class, what type of members does the interface add?

A Private members

B Static members

C Constructor parameters

D Instance members

Merged interfaces add instance members to the class type.

If a class Car is merged with an interface adding a property color, what must the class do?

A Ignore the <code>color</code> property

B Implement the <code>color</code> property

C Rename the class

D Add a static <code>color</code> property

The class must implement all properties declared in the merged interface.

Can merging interfaces add static members to a class type directly?

A Yes, always

B Only if the interface uses the static keyword

C No, static members require merging with the constructor type

D Only if the class is abstract

Static members require merging with the class constructor type, not the instance interface.

Why might you merge an interface with a class from a third-party library?

A To add extra type information without changing the original code

B To rename the class

C To remove properties from the class

D To convert the class to a function

Merging interfaces lets you extend types safely without modifying external code.

Explain how merging a class with an interface works in TypeScript and why it is useful.

Think about how interfaces and classes combine by name and what that means for the class type.

Describe the difference between adding instance members and static members when merging interfaces with classes.

Consider what part of the class the interface affects.