Merge multiple queries in Firestore

MergeQuery.ts

export type CompareFunction = (a: object, b: object) => number;
export type MergeQueryObserver = (docs: DocumentData[]) => void;
export type MergeErrorObserver = (
  e: Error | string | object,
  queryPosition: number
) => void;

export class MergeQuery {
  private observers: Set<{ fn: MergeQueryObserver; err: MergeErrorObserver }>;
  private compareFunction = defaultCompareFunction;
  private docs: DocumentData[][] = [[], []];
  private queryCancelers: Function[] = [];

  constructor(...queries: Query[]) {
    this.observers = new Set();
    queries.forEach((query, position) => {
      this.queryCancelers.push(
        query.onSnapshot(
          snap => {
            this.updateDocs(snap.docs, position);
          },
          e => this.err(e, position)
        )
      );
    });
  }

  setCompareFunction(compareFx: CompareFunction): void {
    this.compareFunction = compareFx;
  }

  /**
   * Listen for updates to the merged data. Receives the entire array after each update.
   *
   * @param observer receives an array of data objects sorted using compareFunction.
   * @return an unsubscribe function
   */
  subscribe(
    observer: MergeQueryObserver,
    errorObserver?: MergeErrorObserver
  ): () => void {
    const obs = {
      fn: observer,
      err:
        errorObserver ||
        (() => {
          /* do nothing */
        })
    };
    this.observers.add(obs);
    return () => {
      this.observers.delete(obs);
    };
  }

  destroy() {
    this.queryCancelers.forEach(fn => fn());
    this.queryCancelers.length = 0;
    this.docs.length = 0;
    this.observers.clear();
  }

  private err(e: Error | object, pos: number) {
    this.observers.forEach(obs => {
      if (obs.err) obs.err(e, pos);
    });
  }

  private updateDocs(docs: DocumentData[], position: number) {
    this.docs[position] = docs.map(dd => ({ $id: dd.id, ...dd.data() }));
    const sortedData = [...new Set(this.docs.flat())].sort(
      this.compareFunction
    );
    this.observers.forEach(obs => obs.fn(sortedData));
  }
}