PDF.js version: 3.5.141

These two properties should be marked as not mandatory as they are initialized if not present
if (this._spreadMode === mode) {
      return; 
    }
    if (!isValidSpreadMode(mode)) {
      throw new Error(`Invalid spread mode: ${mode}`);
    }
    this._spreadMode = mode;
    this.eventBus.dispatch("spreadmodechanged", { source: this, mode });
    this._updateSpreadMode( this._currentPageNumber);
  }
  _updateSpreadMode(pageNumber = null) {
    if (!this.pdfDocument) {
      return;
    }
    const viewer = this.viewer,
      pages = this._pages;
    if (this._scrollMode === ScrollMode.PAGE) {
      this.#ensurePageViewVisible();
    } else {
      
      viewer.textContent = "";
      if (this._spreadMode === SpreadMode.NONE) {
        for (const pageView of this._pages) {
          viewer.append(pageView.div);
        }
      } else {
        const parity = this._spreadMode - 1;