src/utils/combineReducers.js
function getErrorMessage(key, action) {
  var actionType = action && action.type;
  var actionName = actionType && `"${actionType.toString()}"` || 'an action';

  return (
    `Reducer "${key}" returned undefined handling ${actionName}. ` +
    `To ignore an action, you must explicitly return the previous state.`
  );
}
function getErrorMessage(key, action) {
  var actionType = action && action.type;
  var actionName = actionType && `"${actionType.toString()}"` || 'an action';

  return (
    `Reducer "${key}" returned undefined handling ${actionName}. ` +
    `To ignore an action, you must explicitly return the previous state.`
  );
}
/* eslint-disable no-console