Expand external party connection

After reading a tutorial on extending interfaces in TypeScript, I learned that it's possible to extend a third-party library interface by simply defining another interface with the exact same name. The example provided in the tutorial demonstrated this concept:

// third-party source
interface jQueryElement {
    data(name: string): any;
    data(name: string, data: any): jQueryElement;
}

// my code
interface jQueryElement {
    todo(): Todo;
    todo(todo: Todo): void;
}

However, one question still remains: How do you import these extended interfaces without causing name collisions?

As someone who has only worked with TypeScript within Angular projects and not standalone, I'm curious if I can use the same approach or if I should define a new interface using extends like so:

interface jQueryElement2 extends jQueryElement {
    todo(): Todo;
    todo(todo: Todo): void;
}

Answer №1

One way to extend interfaces within the same scope is by declaring them in separate files. For instance, when using JQuery, which declares its interfaces in the global scope, you can easily add additional methods or properties without any issues.

To achieve this, simply create a new file named jquery-extension.d.ts in your project and redeclare the interface with the desired changes. TypeScript will automatically merge the two type declarations into one cohesive definition.

However, it's important to note that merely declaring the methods won't suffice; you must also provide their implementation elsewhere (not in the .d.ts file) to avoid runtime errors during execution.

For example, in the jquery-extension.d.ts file:

interface JQueryStatic {
    customMethod(): void;
}

In the corresponding implementation file, say jquery-extension.ts:

jQuery.fn.customMethod = function() {
    // Implementation logic here
}

If you attempt to use the method without extending the interface, TypeScript will flag an error indicating that customMethod is not recognized within the JQueryStatic interface.

To resolve this, ensure to import jquery-extension.ts in your application's main entry point:

import 'jquery-extension'

That's it! The ambient declaration files like .d.ts don't require explicit imports as the compiler is capable of detecting them on its own.

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