What are some use cases for declaration merging in TypeScript?

What are some use cases for declaration merging in TypeScript?

Declaration merging in TypeScript allows you to combine multiple declarations into a single definition. This is useful for extending interfaces, adding properties to existing types, and enhancing module declarations.

Can you give me an example of extending an interface?

Sure! For instance, if you have an interface User and you want to add a new property age, you can declare it again like this:

interface User {
  name: string;
}

interface User {
  age: number;
}

Now, User has both name and age properties.

What about merging module declarations?

Great question! You can merge module declarations to add new functions or properties to existing modules. For example:

declare module 'my-module' {
  export function newFunction(): void;
}

This adds newFunction to the my-module without modifying the original module.

Are there any limitations to declaration merging?

Yes, there are some limitations. For instance, merging types with conflicting properties can lead to errors. Also, not all types can be merged, such as enums.