higher-order-reducers/src/merge-states.js

A library of simple everyday reducer utility functions

import isPlainObject from 'is-plain-object';
import assign from 'object-assign';

const mergeStates = (reducer) => {
  if (typeof reducer !== 'function') {
    throw new Error('Argument supplied to "mergeStates" is not a reducer');
  }

  return (state, action) => {
    if (!isPlainObject(state)) {
      const type = typeof state;
      throw new Error(`"mergeStates" only works with plain object states, but got a "${type}"`);
    }
    const nextState = reducer(state, action);
    if (!isPlainObject(nextState)) {
      const type = typeof nextState;
      throw new Error(`The reducer in "mergeStates" did not return a plain object. Instead it was a "${type}"`);
    }
    return assign({}, state, nextState);
  };
};

export default mergeStates;