The template for creating a custom reducer.

reducer.ts

export interface State {
  /* Type your state definition. */
}

export const initialState = (): State => ({
  /* Type your initial state. */
});

const reducers = {
  /* Type your reducer by key of the action name. */
  HELLO: (state: State, _action: {}) => state,
} as const;

type Reducers = typeof reducers;
type Payload<K extends keyof Reducers> = Reducers[K] extends (
  state: State,
  action: infer A
) => State
  ? A
  : never;

export type Action = {
  [K in keyof Reducers]: [K, Payload<K>];
}[keyof Reducers];

export type Dispatcher = (action: Action) => void;

export const reducer = (
  state: State,
  [kind, payload]: Action,
): State => reducers[kind](state, payload as Payload<typeof kind>);