Sample typescript redux store

store-sample.ts

export interface SampleState {
  num: number;
}

export enum SampleActionType {
  IncrementNum = "Sample_Increment",
  DecrementNum = "Sample_Decrement"
}

export type SampleAction =
  | { type: SampleActionType.IncrementNum; by: number }
  | { type: SampleActionType.DecrementNum };

export function sampleReducer(s: SampleState | undefined, a: SampleAction): SampleState {
  if (s === undefined) {
    return { num: 100 };
  }

  switch (a.type) {
    case SampleActionType.IncrementNum:
      return { num: s.num + a.by };
    case SampleActionType.DecrementNum:
      return { num: s.num - 1 };
    default:
      return s;
  }
}