AsyncIterableIterator for IndexedDB object stores and indexes

idb-iterator.ts

/**
 * Asyncronous iterable iterator for IndexedDB object stores or indexes
 *
 * @author Florian Lentsch <[email protected]>
 * @license MIT
 *
 * ____________________________________________________________________________
 * REQUIREMENTS: You may have to add "lib": ["esnext.asynciterable"] to
 * your tsconfig.json - see https://stackoverflow.com/questions/43694281/ts2318-cannot-find-global-type-asynciterableiterator-async-generator?answertab=votes#tab-top
 *
 * ____________________________________________________________________________
 * USAGE SAMPLE (Based on the samples at https://developer.mozilla.org/en-US/docs/Web/API/IndexedDB_API/Using_IndexedDB):
 *
 * const customersStore = db.transaction("customers", "readwrite").objectStore("customers");
 * // List all customers:
 * const allCustomers = new IDBIterator(customersStore);
 * for await (let customer of allCustomers) {
 *  console.log("Customer", customer);
 * }
 *
 * // List Donnas:
 * const customersCalledDonna = new IDBIterator(
 *   customersStore.index("name"),
 *   IDBKeyRange.only("Donna")
 * );
 * for await (let customer of customersCalledDonna) {
 *  console.log("Customer called 'Donna'", customer);
 * }
 *
 *
 */
export class IDBIterator implements AsyncIterableIterator<any> {
  private cursorRequest: IDBRequest;
  private cursor: IDBCursorWithValue;
  private curResolver: (result: IteratorResult<any>) => void;

  constructor(private objectStore: IDBObjectStore | IDBIndex, private keyRange: IDBKeyRange = null) { }

  [Symbol.asyncIterator](): AsyncIterableIterator<any> {
    return this;
  }

  public next(): Promise<IteratorResult<any>> {
    return new Promise<IteratorResult<any>>(async resolve => {
      // We need to store the resolver as an instance variable, since else the
      // success callback would always try to resolve the promise of the first
      // next() call only:
      this.curResolver = resolve;

      if (!this.cursorRequest) {
        // Initial request -> Open the cursor and listen for subsequent success events:
        this.cursorRequest = this.objectStore.openCursor(this.keyRange);
        this.cursorRequest.onsuccess = (e) => {
          this.cursor = <IDBCursorWithValue>(<any>e.target).result;
          if (this.cursor) {
            this.curResolver({ value: this.cursor.value, done: false });
          }
          else {
            // We have reached the end:
            this.curResolver({ value: null, done: true });
          }
        };
      }
      else {
        // 2nd request or later -> continue (Still, the above success
        // listener will be called when the cursor has been moved forward):
        this.cursor.continue();
      }
    });
  }
}