EventEmitter types still broken on Typescript 3.9 RC

Typescript 3.9, which should be out some time this week, still has an error when compiling

Here’s my tsconfig:

{
    "compilerOptions": {
        "strict": true,
        "lib": ["esnext", "dom"],
        "types": []
    }
}

And my index.ts:

import eventemitter3 = require("eventemitter3");

The error is here:

  export type EventListener<
    T extends ValidEventTypes,
    K extends EventNames<T>
  > = T extends string | symbol
    ? (...args: any[]) => void
    : (...args: ArgumentMap<T>[K]) => void; // error

“Type ‘K’ cannot be used to index type ‘ArgumentMap<T>’.”

I can’t tell what’s going wrong from a quick glance at the types; there are too many layers of indirection here. My guess is that the compiler just can’t find a declaration that says K is a legal key of ArgumentMap<T> — but I couldn’t figure out what declaration would work.

Discovered in the Typescript nightly tests, where we compile our nightly against the latest shipped types of widely used packages: https://github.com/microsoft/TypeScript/pull/38405

[sandersn]

That works for me, and is basically the workaround discussed in the PR that introduced the breaking change, which means that it should be a pretty safe change.

[gfmio]

I think I’ve found a solution by using Exclude.

  export type ValidEventTypes =
    | string
    | symbol
    | { [K in string | symbol]: any[] | ((...args: any[]) => void) };

  export type EventNames<T extends ValidEventTypes> = T extends string | symbol
    ? T
    : keyof T;

  export type ArgumentMap<T extends object> = {
    [K in keyof T]: T[K] extends (...args: any[]) => void
      ? Parameters<T[K]>
      : T[K] extends any[]
      ? T[K]
      : any[];
  };

  export type EventListener<
    T extends ValidEventTypes,
    K extends EventNames<T>
  > = T extends string | symbol
    ? (...args: any[]) => void
    : (...args: ArgumentMap<Exclude<T, string | symbol>>[K]) => void;

Can you verify that this works for you as well and that this won’t cause any unintended side effects?