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, or merging namespaces.

Can you give me an example of extending an interface?

Sure! For instance, if you have an interface Person with properties name and age, you can create another declaration to add a gender property like this:

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

interface Person {
    gender: string;
}

Now, Person has name, age, and gender.

What about merging namespaces?

Great question! You can merge namespaces to group related functionalities. For example:

namespace Utilities {
    export function log(message: string) {
        console.log(message);
    }
}

namespace Utilities {
    export function error(message: string) {
        console.error(message);
    }
}

Now, Utilities has both log and error functions.

Are there any limitations to declaration merging?

Yes, there are some limitations. For instance, you cannot merge classes or enums, and merging can lead to confusion if not managed properly, especially in large codebases.