https://github.com/mozilla/pdf.js/blob/0df1a5661958df0c4ed2f73b0f3266d825c5c280/src/display/api.js#L710-L719
The return type signature `@returns {Promise<{num: number, gen: number}>}` is mismatched with its definition. (should be `@returns {Promise<number>}`)
if (this._useRequestAnimationFrame) {
        window.requestAnimationFrame(() => {
          this._nextBound().catch(this._cancelBound);
        });
      } else {
        Promise.resolve().then(this._nextBound).catch(this._cancelBound);
      }
    }
    async _next() {
      if (this.cancelled) {
        return;
      }
      this.operatorListIdx = this.gfx.executeOperatorList(
        this.operatorList,
        this.operatorListIdx,
        this._continueBound,
        this.stepper
      );
      if (this.operatorListIdx === this.operatorList.argsArray.length) {
        this.running = false;