TIL: Type alias augmentation in TypeScript

The first thing to remember is type alias augmentation is not allowed. This means we need to use interfaces. I was trying to represent string literals using interfaces but I failed as you guess.

After hours of fighting with TS, I finally found a solution in GitHub.

It is hacky but works... Just make your type alias depends on keyof SomeInterface.

Solution 1

This was the original solution from GitHub. It only supports index types since we use keyof operator.

Declaration:

// packages/core/store/locale/locale.types.ts

export const CHANGE_LOCALE = "CHANGE_LOCALE";

export interface LocaleRegistry {}
export type Locale = keyof LocaleRegistry; // keys of LocaleRegistry

export interface LocaleAction {
  type: typeof CHANGE_LOCALE;
  payload: Locale;
}

export type LocaleState = Locale;

Augmentation:

// apps/admin/src/@types/akdeniz-core.d.ts

import "@akdeniz/core";

declare module "@akdeniz/core" {
  export interface LocaleRegistry {
    // write here your string literals
    tr;
    en;
  }
}

Solution 2

This one supports any type since we rely on values.

Declaration:

// packages/core/store/locale/locale.types.ts

export const CHANGE_LOCALE = "CHANGE_LOCALE";

export interface LocaleRegistry {}
export type Locale = LocaleRegistry[keyof LocaleRegistry]; // values of LocaleRegistry

export interface LocaleAction {
  type: typeof CHANGE_LOCALE;
  payload: Locale;
}

export type LocaleState = Locale;

Augmentation:

// apps/admin/src/@types/akdeniz-core.d.ts

import "@akdeniz/core";

declare module "@akdeniz/core" {
  export interface LocaleRegistry {
    // write here any type as value
    0: "tr";
    1: "en";
    2: number[];
    someRandomKey: Uint16Array;
  }
}

Or even better with helper interface Registry:

// packages/core/src/index.ts

// ...

export interface Registry<T> {
  readonly [n: number]: T;
}

// ...

// apps/admin/src/@types/akdeniz-core.d.ts

import { Registry } from "@akdeniz/core";

declare module "@akdeniz/core" {
  export interface LocaleRegistry extends Registry<"en" | "tr" | number[] | Uint16Array> {}
}