Google Apps Script to Center Selection Horizontally and Vertically on the Slide

CenterSelectionHorizontallyAndVerticallyOnSlide.gs

/**
 * @OnlyCurrentDoc
 */

/**
 * Create a open translate menu item.
 * @param {Event} event The open event.
 */
function onOpen(event) {
  SlidesApp.getUi().createAddonMenu()
      .addItem('Align Selection Centrally on Slide', 'alignSelectedCentrallyOnSlide')
      .addToUi();
}

/**
 * Open the Add-on upon install.
 * @param {Event} event The install event.
 */
function onInstall(event) {
  onOpen(event);
}

function alignSelectedCentrallyOnSlide() {
  var selection = SlidesApp.getActivePresentation().getSelection();
  if (selection == null) {
    return;
  }
  
  per = selection.getPageElementRange();
  if (per == null) {
    return;
  }
  
  pes = per.getPageElements();
  if (pes == null || pes.length == 0) {
    return;
  }
  
  pes.forEach(function(f, i) { 
    pes[i].alignOnPage(SlidesApp.AlignmentPosition.HORIZONTAL_CENTER);
    pes[i].alignOnPage(SlidesApp.AlignmentPosition.VERTICAL_CENTER);
  });
  
}