TypeScript has encountered an issue where a specific type A cannot be assigned to another type A, even though

Encountering a Typescript issue where it claims that type A is not compatible with type A, even though they are identical:

Check out this playground link for the code snippet in context:

declare interface ButtonInteraction {
  user: any;
  customId: string;
  reply: (message: string) => void
}

enum TranslatorLangs {
  FR = 'fr',
  EN = 'en',
}

export class UserService {
  public async setUserLocale(user: any, locale: TranslatorLangs): Promise<any> {
    user.locale = locale;
  }
}

export interface ButtonHandlerInterface {
  commandName: string;
  handle<T = unknown>(interaction: ButtonInteraction, value: T): void | Promise<void>
}

export default class MbtiCommandHandler implements ButtonHandlerInterface {
  public commandName = 'setLocale';

  private userService: UserService = new UserService();

  async handle<TranslatorLangs>(interaction: ButtonInteraction, value: TranslatorLangs): Promise<void> {
    this.userService.setUserLocale(interaction.user, value);
    //                                               ^^^^^
    //                         Argument of type 'TranslatorLangs' is not assignable to parameter of type 'TranslatorLangs'
    interaction.reply(`Ok, so it will be ${interaction.customId}`);

    return interaction.reply('ok');
  }
}

Answer №1

They are not exactly identical, as the term "TranslatorLangs" is used in two distinct ways. First, it serves as an enum, and second, it represents the generic parameter that can be accepted. To differentiate between these uses, let's modify the handle function as follows:

async handle<T>(interaction: ButtonInteraction, value: T): Promise<void>

This code snippet indicates that handle is a generic function where "T" acts as a placeholder for the type being passed into it. It may be TranslatorLangs or any other data type like boolean. Therefore, attempting to pass a boolean into setUserLocale would result in an error.

It seems like you intend to make ButtonHandlerInterface itself generic, which can be done through the following definition:

export interface ButtonHandlerInterface<T = unknown> {
  commandName: string;
  handle(interaction: ButtonInteraction, value: T): void | Promise<void>
}

You can implement it like this:

export default class MbtiCommandHandler implements ButtonHandlerInterface<TranslatorLangs> {
   // ...
  async handle(interaction: ButtonInteraction, value: TranslatorLangs): Promise<void> {
  // ...

Playground link

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