Guide on utilizing interface within a class for functional de-structuring in TypeScript

In my current project, I have a Class that contains a method which I would like to utilize in a de-structured manner, primarily due to the presence of multiple optional arguments. To maintain strict typing, I have created an Interface to define these arguments.

Although the current setup is functional, it can be a bit cumbersome, especially when most functions require de-structured arguments.

My query is: Can we define an interface within a class?

The motivation behind this is simply to enhance code readability and maintenance. In larger classes, I find myself having to scroll outside the class to modify a "function" interface, whereas having the interface alongside the function would be much more organized.

Here's an example of the current setup that works:


interface LoadUsersInterface {
  status: 'active' | 'inactive',
  options?: QueryOptions
}

class UserService {
  loadUsers({ status, options }: LoadUsersInterface) {
    ...
  }  
}

And here's what I envision and would like to achieve:


class UserService {
 
  interface LoadUsersInterface {
    status: 'active' | 'inactive',
    options?: QueryOptions
  }

 loadUsers({ status, options }: LoadUsersInterface) {
    ...
  }  
}

Note: This scenario is happening within an Angular 11 application. While there may be specific features in Angular that I'm unaware of, I believe this is more of a JavaScript / TypeScript related question.

Answer №1

Currently, it is not possible to define types or interfaces directly within a class. This limitation has been addressed in a feature request on GitHub, which you can find here. The request discusses the need for declaring types alongside the methods that utilize them, but the issue is quite old and marked for "Revisit". While there is a possibility of this feature being added in the future, it is uncertain.

In the meantime, there are workarounds available. If you require a named type, it is recommended to declare it outside the class as you currently do, even though it may involve some scrolling.

For cases where a type is used only once, an anonymous type can be directly specified within the method:

class UserService {    
    loadUsers({ status, options }: {
        status: 'active' | 'inactive',
        options?: QueryOptions
    }) {
        // implementation
    }
}

However, this method may not be as neat as desired. Nevertheless, it is a viable option for now.

Click here to view code in the TypeScript Playground

Answer №2

One way to define the interface is directly in the method signature, eliminating the need for an additional file. However, this approach may become difficult to read as the list of parameters grows.

class UserService { 
 loadUsers(payload: {status: 'active' | 'inactive', options?: QueryOptions}) {
    console.log(payload.status);
  }  
}

Check out the TypeScript play to see this in action

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