blog-post-introduction-to-redux-for-web-application-development-04.js

function incrementHandler(state, action) {
  var nextState = Object.assign({}, state);
  nextState.counter += 1;
  return nextState;
}

function decrementHandler(state, action) {
  var nextState = Object.assign({}, state);
  nextState.counter -= 1;
  return nextState;
}