Testing Reducers with Typed Actions

import { createStore } from 'redux';
import { expect } from 'chai';

// Action types
const INCREMENT = 'INCREMENT';
const DECREMENT = 'DECREMENT';

// Typed actions
interface IncrementAction {
  type: typeof INCREMENT;
}

interface DecrementAction {
  type: typeof DECREMENT;
}

type CounterAction = IncrementAction | DecrementAction;

// Action creators
function increment(): IncrementAction {
  return { type: INCREMENT };
}

function decrement(): DecrementAction {
  return { type: DECREMENT };
}

// Reducer
function counterReducer(state: number = 0, action: CounterAction): number {
  switch (action.type) {
    case INCREMENT:
      return state + 1;
    case DECREMENT:
      return state - 1;
    default:
      return state;
  }
}

// Testing the reducer
describe('counterReducer', () => {
  it('should increment the state by 1', () => {
    const initialState = 0;
    const state = counterReducer(initialState, increment());
    expect(state).to.equal(1);
  });

  it('should decrement the state by 1', () => {
    const initialState = 1;
    const state = counterReducer(initialState, decrement());
    expect(state).to.equal(0);
  });

  it('should not affect state for unknown action', () => {
    const initialState = 2;
    const state = counterReducer(initialState, { type: 'UNKNOWN_ACTION' } as CounterAction);
    expect(state).to.equal(2);
  });
});