EventEmitter types still broken on Typescript 3.9 RC

[sandersn]

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'."

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: microsoft/TypeScript#38405

[gfmio]

Thanks, @sandersn. I will have a look, but doesn't this point to a regression in TS 3.9, given that it seems to work in TS 3.8 (and below)?

[gfmio]

@sandersn I just tried this with typescript versions 3.9.0-beta, 3.9.1-rc, 3.9.2-insiders.20200509 and 4.0.0-dev.20200511 and I cannot reproduce this error. Have you tried your test with the latest version [email protected]?

[sandersn]

@gfmio I can repro this by running

$ tsc index.d.ts

From the root of the eventemitter3 repo. Can you double-check that your globally installed version of typescript is 3.9 RC or higher? (I haven't tried with 3.9.0-beta, just 3.9.1-rc and 4.0.0-dev)

[sandersn]

I'll take another look at the types and see why they broke in 3.9.

[gfmio]

@sandersn I think that this is due the conditional type I'm using in ValidEventTypes:

  export type ValidEventTypes<T = any> = string | symbol | T extends {
    [K in keyof T]: any[] | ((...args: any[]) => void);
  }
    ? T
    : never;

  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<T>[K]) => void;

As I'm removing string | symbol using the conditional type in EventListener, the problem lies with type ValidEventTypesWithoutStringOrSymbol<T = any> = T extends { [K in keyof T]: any[] | ((...args: any[]) => void); } ? T : never.

This conditional type represents an object with arbitrary keys whose keys are all any[] or (...args: any[]) => void). In ts3.8 and below, this type is compatible with object, but it is not in 3.9 and above.

[sandersn]

The lack of errors in 3.8 is due the behaviour of T extends any in 3.9 -- it no longer suppresses errors but behaves like you actually wrote T extends unknown. This makes Typescript more sound in obscure cases. In the PR we discussed a mechanical workaround, but that just puts off the problem.

In this case T extends ValidEventTypes<any> is equivalent to any, so in EventListener (and everywhere else), ValidEventTypes is currently a roundabout way of saying any.

I'm trying to figure out a way to get rid of the error, but in the meantime, this is closer to your prose description of ValidEventTypes:

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

[gfmio]

@sandersn Thanks for the heads up. I've thought about it and can't we just do the following?

export type ValidEventTypes = string | symbol | Record<any, any[] | ((...args: any[]) => void)>;

[sandersn]

Yes, that seems to work, although I'm suspicious, because it should be equivalent to my attempt, which still has the 3.9 error. I'm not sure why that is. @weswigham do you have any ideas?

[weswigham]

The Record over any also produces a numeric index signature, and, if related to another Record, specifically, will admit behavior similar to a symbol index signature (since mapped types are compared by their indexes as a whole).

[gfmio]

So, the valid key types that the Record should really accept are string | symbol, since number is not officially supported by eventemitter3 afaik. However, if I put in anything other than any in the Record's key type argument, the type error returns.

[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?

[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.