Nodemcu v3 esp8266 module problem

Just add const LED1 = D2 or const LED1 = NodeMCU.D4; //(pin2 of ESP8266) to get the build-in LED blinking.
And actually you don't need the variables LED1 and on as you can simply call D2.toggle() or NodeMCU.D4.toggle() to toggle the pin D2/build-in LED:

setInterval(function() {
   D2.toggle();
}, 1000);

To stop the blinking call clearInterval().