ReduxExample.js

// library code
function createStore(reducer) {
    // Store should have parts
    // The state
    // Get the  state
    // Listen to changes on state
    // Update the state


    let state

    let listeners = []

    const getState = () => state

    const subscribe = (listener) => {
        listeners.push(listener)
        return () => {
            listeners = listeners.filter(l => l != listener)
        }
    }
    const dispatch = (action) => {
        state = reducer(state, action);
        listeners.forEach(listener => listener())
    }

    return {
        getState,
        subscribe,
        dispatch
    }
}


function todos(state = [], action) {
    switch (action.type) {
        case 'ADD_TODO':
            return state.concat([action.todo])
        case 'REMOVE_TODO':
            return state.filter(todo => todo.id != action.id)
        case 'TOGGLE_TODO':
            return state.map(todo => todo.id != action.id ? todo : Object.assign({}, todo, {
                complete: !todo.complete
            }))
        default:
            return state
    }
}


function goals(state = [], action) {
    switch (action.type) {
        case 'ADD_GOAL':
            return state.concat([action.goal])
        case 'REMOVE_GOAL':
            return state.filter((goal) => goal.id !== action.id)
        default:
            return state
    }
}

function app(state = {}, action) {
    return {
        todos: todos(state.todos, action),
        goals: goals(state.goals, action),
    }
}

const store = createStore(app)


store.subscribe(() => console.log("State is", store.getState()));

store.dispatch({
    type: 'ADD_TODO',
    todo: {
        id: 0,
        name: 'Master React',
        complete: false,
    }
})

store.dispatch({
    type: 'ADD_TODO',
    todo: {
        id: 1,
        name: 'Learn Redux',
        complete: false,
    }
})

store.dispatch({
    type: 'ADD_TODO',
    todo: {
        id: 2,
        name: 'Go to the gym',
        complete: true,
    }
})

store.dispatch({
    type: 'REMOVE_TODO',
    id: 1
})

store.dispatch({
    type: 'TOGGLE_TODO',
    id: 0
})

store.dispatch({
    type: 'ADD_GOAL',
    goal: {
        id: 0,
        name: 'Learn Redux'
    }
})

store.dispatch({
    type: 'ADD_GOAL',
    goal: {
        id: 1,
        name: 'Lose weight'
    }
})

store.dispatch({
    type: 'REMOVE_GOAL',
    id: 1
})