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TypeScript type definitions: how to declare class constructors

I have a home-brew JavaScript library (one js file) that contains a number of classes. In order to use it in my TypeScript projects, I have created a d.ts file that contains type definitions for all those classes. A typical definition would look like this:

export interface ClassExample {
  new(param: boolean): ClassExample;
  propertyName: number|string;
  log(...args: any[]): void;
}

And it works well. But I am getting bashed by linter rule @typescript-eslint/no-misused-new: "Interfaces cannot be constructed, only classes". Of course, I can configure my linter and/or add exceptions for the particular d.ts file, but this makes me contemplate if there should be some other approach to describe my classes. Should an abstract class declaration be used instead? I tried to look up how the class constructors are declared for publicly available Node.js modules, and found this code in @types/node/console.d.ts file:

interface ConsoleConstructor {
  prototype: Console;
  new(stdout: WritableStream, stderr?: WritableStream, ignoreErrors?: boolean): Console;
  new(options: ConsoleConstructorOptions): Console;
}

So, I figure, declaring class constructors in interface is not a grave sin, if the authors of Node.js are practicing it?

over 4 years ago · Santiago Trujillo

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Answer 1 · 2022-04-25T17:02:38.349Z

According to the handbook, you'd declare that class with the declare keyword, like this:

declare class ClassExample {
    constructor(param: boolean);
    propertyName: number|string;
    log(...args: any[]): void;
}

Here's the section linked above as it appears as of when this answer was written (my emphasis):

Classes

Documentation

You can create a greeter by instantiating the Greeter object, or create a customized greeter by extending from it.

Code
const myGreeter = new Greeter("hello, world");
myGreeter.greeting = "howdy";
myGreeter.showGreeting();

class SpecialGreeter extends Greeter {
  constructor() {
    super("Very special greetings");
  }
}
Declaration

Use declare class to describe a class or class-like object. Classes can have properties and methods as well as a constructor.
declare class Greeter {
  constructor(greeting: string);

  greeting: string;
  showGreeting(): void;
}

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TypeScript type definitions: how to declare class constructors

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Andres GPT