packages/node_modules/pouchdb-utils/src/upsert.js

// this is essentially the "update sugar" function from daleharvey/pouchdb#1388
// the diffFun tells us what delta to apply to the doc.  it either returns
// the doc, or false if it doesn't need to do an update after all
function upsert(db, docId, diffFun) {
  return new Promise(function (fulfill, reject) {
    db.get(docId, function (err, doc) {
      if (err) {
        /* istanbul ignore next */
        if (err.status !== 404) {
          return reject(err);
        }
        doc = {};
      }

      // the user might change the _rev, so save it for posterity
      var docRev = doc._rev;
      var newDoc = diffFun(doc);

      if (!newDoc) {
        // if the diffFun returns falsy, we short-circuit as
        // an optimization
        return fulfill({updated: false, rev: docRev});
      }

      // users aren't allowed to modify these values,
      // so reset them here
      newDoc._id = docId;
      newDoc._rev = docRev;
      fulfill(tryAndPut(db, newDoc, diffFun));
    });
  });
}

function tryAndPut(db, doc, diffFun) {
  return db.put(doc).then(function (res) {
    return {
      updated: true,
      rev: res.rev
    };
  }, function (err) {
    /* istanbul ignore next */
    if (err.status !== 409) {
      throw err;
    }
    return upsert(db, doc._id, diffFun);
  });
}

export default upsert;