Storage.js

class Storage {
  constructor(dbName, remoteDbUrl) {
    this.dbName = dbName
    this.remoteDbUrl = remoteDbUrl;
  }

  init() {
    this.database = new manager(`http://127.0.0.1:5984/`, this.dbName);

    return this.database.createDatabase().then(res => {
      if (!res.ok && (res.status !== 412 || res.error !== 'file_exists')) {
        throw new Error(`Error connecting to Storage: ${JSON.stringify(res)}, ${JSON.stringify(this)}`);
      }
    });
  };

  getAll() {
    return this.database.getAllDocuments();
  };

  get(key) {
    return this.database.getDocument(key);
  };

  startReplication(source, target, createTarget) {
    return this.database.replicate(source, target, true, createTarget).then((res) => {
      if (!res.session_id) {
        throw new Error(JSON.stringify(res));
      }
      return res.session_id;
    });
  };

  /**
   * @returns {Promise} a promise with the event listener that the caller can then subscribe to.
   * @throws if the change feed cannot be initialized (eg. last_seq cannot be retrieved from the database)
   * For proper use with React Native see https://colinramsay.co.uk/2015/07/04/react-native-eventemitters.html
   * Emits two possible events:
   *  * 'change' with JSON parsed result from change feed as argument
   *  * 'error' with last error or bad response Errors will be logged automatically to console
   *  After an error there is delay before listening again with exponential backoff.
   */
  getChangesEventEmitter() {
    if (!this.changesEventEmitter) {
      this.changesEventEmitter = new EventEmitter();
      this.changesRetryCount = 0;
      return this.database.latestRevision().then(lastRevision => {
        this.lastSequence = lastRevision;
        this.listenForChange();
        return this.changesEventEmitter;
      })
    } else {
      return Promise.resolve(this.changesEventEmitter);
    }
  };

  /**
   * Override the default change feed options with new options
   * See See Query parameters in http://developer.couchbase.com/documentation/mobile/1.2/develop/references/couchbase-lite/rest-api/database/get-changes/index.html
   * @returns void
   */
  setChangesFeedOptions(options) {
    this.changeFeedOptions = options;
  };

  getChangesFeedOptions() {
    return Object.assign({feed: 'longpoll'}, this.changeFeedOptions || {}, {since: this.lastSequence});
  };

  listenForChange() {
    const params = this.getChangesFeedOptions();
    console.log('Listening for changes with', params);
    return this.database.getChanges(params).then(res => {
      if (typeof(res.last_seq) === 'number') {
        this.lastSequence = res.last_seq;
        this.changesRetryCount = 0;
        this.changesEventEmitter.emit(CHANGE_EVENT_TYPE, res);
      } else {
        throw new Error(JSON.stringify(res));
      }
      return this.listenForChange();
    }).catch((error) => {
      this.changesRetryCount += 1;
      console.log('ERROR Listening for changes, RetryCount=', this.changesRetryCount, 'Error=', error);
      this.changesEventEmitter.emit(ERROR_EVENT_TYPE, error);
      return new Promise((resolve, reject) => {
        setTimeout(resolve, this.retryDelay(this.changesRetryCount));
      }).then(() => {
        return this.listenForChange()
      });
    })
  };

  retryDelay(retryCount = 0) {
    return (Math.exp(retryCount)) + 15 + (Math.random() * 30 * (retryCount + 1))
  };
}