orientation.jxa

const app = Application.currentApplication();
app.includeStandardAdditions = true;
const sp = Application("System Preferences");
const se = Application("System Events");

const findWindow = (systemEvents, displayName) => {
  return se.processes['System Preferences'].windows.byName(displayName);
}

const findPopupButton = (display, name) => {
  return display.tabGroups[0].popUpButtons.byName(name)
}

const rotateDisplay = (systemEvents = se, display, popup, orientation = 'Standard', timeout = 3) => {
  popup.click();
  systemEvents.keystroke(orientation);
  systemEvents.keyCode(36);

  // Wait for screen flash from orientation switch
  delay(timeout);

  // Click orientation confirmation button
  const sheets = display.sheets;

  if (sheets.length) {
    const confirmation = sheets[0].buttons.byName('Confirm');
    return confirmation.enabled() && confirmation.click();
  }
};

// Bring Display Window Forward
sp.activate()
// Bring 'Displays' to the foreground
sp.panes["com.apple.preference.displays"].reveal();


// Find the primary display
const standardDisplay = findWindow(se,'DELL U2717D (1)');
// Find the drop down for orientation
const standardRotation = findPopupButton(standardDisplay, 'Rotation:');
// Rotate
rotateDisplay(se, standardDisplay, standardRotation);

// Find the second display
const rotatedDisplay = findWindow(se, 'DELL U2717D (2)')
// Find the drop down for orientation
const rotatedRotation = findPopupButton(rotatedDisplay, 'Rotation:');

// Click drop down and select orientation by keystroke
rotateDisplay(se, rotatedDisplay, rotatedRotation, '90');

// Clean Up
sp.quit();