npm install --save rpi-hdmi

const hdmi = require('rpi-hdmi');

hdmi.isConnected((err, hdmiState) => {
  if(hdmiState)
    console.log('HDMI is On')
  else 
    console.log('HDMI is off')
});

const hdmi = require('rpi-hdmi');

hdmi.on((err) => {
  if(!err)
    console.log('HDMI is off')
});

const hdmi = require('rpi-hdmi');

hdmi.off((err) => {
  if(!err)
    console.log('HDMI is On')
});