Redux.js

function createReducer(handlers, initialState) {
    return function reducer(state = initialState, action) {
    	return handlers[action.type] ?
    		  handlers[action.type](state, action)
    		: state
	}
}

const math = createReducer({
	INCREMENT: (state) => state + 1,
	DECREMENT: (state) => state - 1
}, 0)