The TS type guard fails to identify a non-empty property in a class object

Encountering an issue with TypeScript that involves a class property typed as AmplitudeFeatureFlags | {} = {}; initialized as an empty object. Attempting to retrieve a value from this class property using a method that takes in a property name argument of enum type, but running into errors as shown below.

Seeking guidance on how to resolve this problem.

export enum AmplitudeFeatureFlagNames {
  addTeamMembersFromUserMenu = 'addTeamMembersFromUserMenu',
  inAppHandRaiser = 'inAppHandRaiser',
}

type AmplitudeFeatureFlagWithPayload<Payload extends object = {}> = {
  key: string;
  payload?: Payload;
};

export type AmplitudeFeatureFlags = {
  [AmplitudeFeatureFlagNames.addTeamMembersFromUserMenu]: AmplitudeFeatureFlagWithPayload;
  [AmplitudeFeatureFlagNames.inAppHandRaiser]: AmplitudeFeatureFlagWithPayload;
};

class User {
  private amplitudeFeatureFlags: AmplitudeFeatureFlags | {} = {};

    getAmplitudeFeatureFlagPayload(flagName: AmplitudeFeatureFlagNames) {
      if (flagName in this.amplitudeFeatureFlags) {
    
    **//ERROR on the line below:**
    //Element implicitly has an 'any' type because expression of type 
   //'AmplitudeFeatureFlagNames' can't be used to index type '{} | 
    //AmplitudeFeatureFlags'.
    //Property '[AmplitudeFeatureFlagNames.addTeamMembersFromUserMenu]' does not exist on 
    //type '{} | AmplitudeFeatureFlags'.

        **return this.amplitudeFeatureFlags[flagName]?.payload;**
      }
  }
}

Answer №1

One reason for the issue is that this.amplitudeFeatureFlags may not always be of type AplitudeFeatureFlags. Your current declaration is as follows:

private amplitudeFeatureFlags: AmplitudeFeatureFlags | {}

This means that TypeScript recognizes it could also be of type {}, resulting in

{}[key: AmplitudeFeatureFlagNames]
being implicitly typed as any.

To address this, you should provide a more specific declaration:

private amplitudeFeatureFlags: Partial<AmplitudeFeatureFlags> = {}

With this change, your code will look like:

export enum AmplitudeFeatureFlagNames {
  addTeamMembersFromUserMenu = 'addTeamMembersFromUserMenu',
  inAppHandRaiser = 'inAppHandRaiser',
}

type AmplitudeFeatureFlagWithPayload<Payload extends object = {}> = {
  key: string;
  payload?: Payload;
};

export type AmplitudeFeatureFlags = {
  [AmplitudeFeatureFlagNames.addTeamMembersFromUserMenu]: AmplitudeFeatureFlagWithPayload;
  [AmplitudeFeatureFlagNames.inAppHandRaiser]: AmplitudeFeatureFlagWithPayload;
};

class User {
  private amplitudeFeatureFlags: Partial<AmplitudeFeatureFlags> = {};

    getAmplitudeFeatureFlagPayload(flagName: AmplitudeFeatureFlagNames): AmplitudeFeatureFlagWithPayload['payload'] | undefined {
      if (flagName in this.amplitudeFeatureFlags) {
        return this.amplitudeFeatureFlags[flagName]?.payload;
      }
      return undefined
  }
}

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