Issue with TypeScript Generics in Apollo Server PubSub Implementation

I’m implementing subscriptions in Apollo Server using TypeScript and a PubSub class for event handling. However, I’ve encountered an issue when trying to use a named interface for the PubSub generic type.

When using a named interface, I encounter the following error:

interface PubSubEvents {
  CHANGE_EVENT: string;
}

export interface Context {
  pubsub: PubSub<PubSubEvents>; 
  // Error: Type 'PubSubEvents' does not satisfy the constraint '{ [event: string]: unknown; }'.
}

Interestingly, the same code works fine when using an inline type definition:

export interface Context {
  pubsub: PubSub<{
    CHANGE_EVENT: string;
  }>;
}

Why does the inline type definition work, but the named PubSubEvents interface fails? Is this a limitation of TypeScript’s type system or an issue with how PubSub is implemented? How can I fix this while keeping the named interface?

>Solution :

You can use:

type PubSubEvents = {
  CHANGE_EVENT: string;
}

or add an explicit index signature:

interface PubSubEvents {
  CHANGE_EVENT: string;

  [event: string]: unknown;
}

References:

• https://github.com/microsoft/TypeScript/issues/42825
• https://github.com/microsoft/TypeScript/issues/15300#issuecomment-332366024