How can I convey to TypeScript that functions are being "mixed in" from a different module?

Let me explain the scenario I am facing:

// Application.ts
import MicroEvent from 'microevent-github'

class Application {
  // stuff...
  something() {
    // It is also saying "trigger" is undefined,
    // but it IS defined, MicroEvent defined it.
    this.trigger('foo')
  }
}

// get the `bind`, `unbind`, and other methods from MicroEvent
MicroEvent.mixin(Application)

const app = new Application()

const handleFoo = () => console.log('foo')

// try to use them, get the squiggly errors saying
// `bind` doesn't exist on Application, etc.
application.bind('foo', handleFoo)
application.unbind('foo', handleFoo)

I have incorporated the MicroEvent into my application, which introduces some new methods to the object. But now, I'm facing issues in VSCode as it claims that bind and unbind are not present on the Application instance... even though they are. How can I make TypeScript recognize this?

Adding the following code did not resolve the problem:

type Application = {
  bind: (eventType: string, callback: () => void) => void
  unbind: (eventType: string, callback: () => void) => void
  trigger: (eventType: string) => void
}

Answer №1

Typescript may not recognize that the mixin is altering the class.

To address this, you can use declare statements in Typescript. These statements inform Typescript about the existence of certain types without specifying their implementation. Although this method lacks type safety checks, it can be useful when working with untyped libraries and no other options are available.

class Application {
    declare bind: (eventType: string, callback: () => void) => void
    declare unbind: (eventType: string, callback: () => void) => void
    declare trigger: (eventType: string) => void

    //...
}

Playground


To make this more reusable, you can create an abstract base class that declares these methods and then inherit from it.

abstract class MicroEventBase {
  declare bind: (eventType: string, callback: () => void) => void
  declare unbind: (eventType: string, callback: () => void) => void
  declare trigger: (eventType: string) => void
}

class Application extends MicroEventBase {
  //...
}

Playground

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