What is TypeScript declaration merging?

Can you explain what TypeScript declaration merging is?

Sure! TypeScript declaration merging allows you to define multiple declarations for the same entity, like interfaces or namespaces, and TypeScript will merge them into a single definition.

Can you give me an example of how declaration merging works?

Absolutely! For instance, if you have two interfaces with the same name, TypeScript will combine their properties into one. Here's a quick example:

interface User {
  name: string;
}

interface User {
  age: number;
}

const user: User = { name: 'Alice', age: 30 };

In this case, the User interface has both name and age properties.

Are there any limitations to declaration merging?

Yes, there are some limitations. For example, only certain types can be merged, like interfaces and namespaces. Classes and enums cannot be merged in the same way.

How can I check if my declarations are merging correctly?

You can check by using TypeScript's type checking features. If you try to access a property that isn't defined in the merged declaration, TypeScript will throw an error, helping you identify issues.