Example Jest Tests

counter.js

import { createStore } from 'redux';

const INCREASE_COUNT = 'INCREASE_COUNT';

// Action Creator using ES5 Arrow Syntax
export const increaseCount = (amount = 1) => ({
    type: INCREASE_COUNT,
    payload: amount,
});

const DEFAULT_STATE = {
    count: 0,
    nightMode: true,
};

export function reducer(state = DEFAULT_STATE, action = {}) {
    switch(action.type) {
        case INCREASE_COUNT:
            return {
                ...state,
                count: state.count + action.payload,
            };
    }

    return state;
}

export const store = createStore(reducer);

counter.spec.js

import { increaseCount, reducer, store } from './counter';

describe('Counter', function() {
    it('should have a default state', function() {
        const result = reducer();
        expect(result.count).toEqual(0);
    });

    it('should increase the count', function() {
        const action = increaseCount();
        const result = reducer(undefined, action);
        expect(result.count).toEqual(1);
    });

    it('should increase the count by custom count value', function() {
        const action = increaseCount(5);
        const result = reducer(undefined, action);
        expect(result.count).toEqual(5);
    });
});

describe('Sheep counting store', function() {
    it('should return the state', function() {
        const state = store.getState();
        expect(state.count).toEqual(0);
    });

    it('should dispatch actions and update the state', function() {
        const action = increaseCount();
        store.dispatch(action);
        expect(store.getState().count).toEqual(1);
    });

    it('should call the subscribers when the store data changes', function() {
        const listener = jest.fn();
        store.subscribe(listener);
        const action = increaseCount();
        store.dispatch(action);
        expect(listener).toHaveBeenCalled();
    });

    it('should not call the unsubscribed components when the store data changes', function() {
        const listener = jest.fn();
        const unsubscribe = store.subscribe(listener);
        const action = increaseCount();
        unsubscribe();
        store.dispatch(action);
        expect(listener).not.toHaveBeenCalled();
    });
});