packages/node_modules/pouchdb-adapter-idb/src/runBatchedCursor.js

// Abstraction over IDBCursor and getAll()/getAllKeys() that allows us to batch our operations
// while falling back to a normal IDBCursor operation on browsers that don't support getAll() or
// getAllKeys(). This allows for a much faster implementation than just straight-up cursors, because
// we're not processing each document one-at-a-time.
function runBatchedCursor(objectStore, startKey, descending, batchSize, onBatch) {

  // Bail out of getAll()/getAllKeys() in the following cases:
  // 1) either method is unsupported (we need both)
  // 2) batchSize is 1 (might as well use IDBCursor), or batchSize is -1 (i.e. batchSize unlimited,
  //    not really clear the user wants a batched approach where the entire DB is read into memory,
  //    perhaps they are filtering on a per-doc basis)
  // 3) descending – no real way to do this via getAll()/getAllKeys()
  var useGetAll = typeof objectStore.getAll === 'function' &&
    typeof objectStore.getAllKeys === 'function' && batchSize > 1 && !descending;

  var keysBatch;
  var valuesBatch;
  var pseudoCursor;

  function onGetAll(e) {
    valuesBatch = e.target.result;
    if (keysBatch) {
      onBatch(keysBatch, valuesBatch, pseudoCursor);
    }
  }

  function onGetAllKeys(e) {
    keysBatch = e.target.result;
    if (valuesBatch) {
      onBatch(keysBatch, valuesBatch, pseudoCursor);
    }
  }

  function continuePseudoCursor() {
    if (!keysBatch.length) { // no more results
      return onBatch();
    }
    // fetch next batch, exclusive start
    range = IDBKeyRange.lowerBound(keysBatch[keysBatch.length - 1], true);
    keysBatch = null;
    valuesBatch = null;
    objectStore.getAll(range, batchSize).onsuccess = onGetAll;
    objectStore.getAllKeys(range, batchSize).onsuccess = onGetAllKeys;
  }

  function onCursor(e) {
    var cursor = e.target.result;
    if (!cursor) { // done
      return onBatch();
    }
    // regular IDBCursor acts like a batch where batch size is always 1
    onBatch([cursor.key], [cursor.value], cursor);
  }

  var range = (startKey && !descending) ? IDBKeyRange.lowerBound(startKey, true) : null;

  if (useGetAll) {
    pseudoCursor = {"continue": continuePseudoCursor};
    objectStore.getAll(range, batchSize).onsuccess = onGetAll;
    objectStore.getAllKeys(range, batchSize).onsuccess = onGetAllKeys;
  } else if (descending) {
    objectStore.openCursor(range, 'prev').onsuccess = onCursor;
  } else {
    objectStore.openCursor(range).onsuccess = onCursor;
  }
}

export default runBatchedCursor;