An example for a helper function to write clean reducers.

mapActionToReducer.js

// Rather than this
function reducer(state, action) {
	switch (action.type) {
		case 'TODO_ADD': {
			return applyAddTodo(state, action)
		}
		case 'TODO_TOGGLE': {
			return applyToggleTodo(state, action)
		}
		default:
			return state
	}
}

// Create a map of action types to reducer.
// Pass that map to a function which returns a reducer of the action type.

const reducer = (reducerOf => (state, action) =>
	(reducerOf[action.type] || (() => state))(state, action))({
	TODO_ADD: applyToggleTodo,
	TODO_TOGGLE: applyToggleTodo,
})

// Make less terse and more resusable with a dedicated mapActionToReducer function

const mapActionToReducer = reducerOf => (state, action) => {
	const reducer = reducerOf[action.type]
	return reducer ? reducer(state, action) : state
}

// Use helper function for clean

const reducer = mapActionToReducer({
	TODO_ADD: applyToggleTodo,
	TODO_TOGGLE: applyToggleTodo,
})