save.js

public save() {
  let db = this.get_db();
  let data = this.get_data();
  let t = this;
  console.log('Save:' + this._id);
  db.get(this._id, {conflicts: false})
    .catch(function (err) {
       if (err.status === 404) {
          return {}; // you can just return an empty map if docs not found
       }
       throw err; // re-throw any other err to propagate
    })
    .then(function (doc) {
      // You probably don't need to handle 409s, unless you're trying to do multiple saves at the same time
      data._rev = doc._rev;
      return db.put(data); // put updated doc, will create new revision
    })
    .then(saveSuccessful)
    .catch(function (err) {
      console.log(err); 
    });
}`