redux-subscribe.js

import get from 'lodash.get';

/**
 * Subscribe partially to redux store changes
 *
 * @example
 *   subscribe(store, "backup.url", (state, prevState) => console.log("change!", state, prevState));
 *
 * @param {Redux.Store} store - Redux store
 * @param {String} path - Path within the store object that should be watched for change. See https://lodash.com/docs/4.17.15#get
 * @param {Function} cb(state, prevState) - Callback for when the store changes at the specified path
 * @returns {void}
 */
const subscribe = (store, path, cb) => {
  let state = get(store.getState(), path);
  store.subscribe(() => {
    const previousState = state;
    state = get(store.getState(), path);
    
    if (previousState !== state) {
      cb(state, previousState);
    }
  });
};