Last but not least, since we want to turn on a relay that will let water flow and then turn it off, the code for turning the water on after one second and off after two seconds is as follows:

var mraa = require('mraa');
var pinD2 = new mraa.Gpio(2);
pinD2.dir(mraa.DIR_OUT);
setTimeout(function() {
  pinD2.write(1);
  setTimeout(function() {
    pinD2.write(0);
  },2000);
},1000);

After a delay of one second, you'll see the  D3 relay module LED turn on and you will also hear a click. This means that the relay connection is closed, and after two seconds it will turn off and open the connection.