Pin - 21 for gpio input and pin 20 is for gpio output if moisture detected.

RaspberryPi_Moisture_Detection.js

/* 
  Install rpi-gpio using 
  npm install rpi-gpio -g
*/
const gpio = require('rpi-gpio');

var pin = 21;
var pin2 = 20;

gpio.setMode(gpio.MODE_BCM);
gpio.setup(pin, gpio.DIR_IN);
gpio.setup(pin2, gpio.DIR_OUT);

function output(status) {
	var mode = (status === false) ? 1 : 0;
	gpio.write(pin2, mode, function(err) {
        if (err) throw err;
        var msg;
        if(status === false){
        		msg = "Moisture detected";
        } else {
        	msg = "Moisture not detected";
        }
        console.log(msg);
	});
}
function test() {
	  gpio.read(pin, (err, data) => {
		   if (err) throw err;
			return output(data);
	  });
}

setInterval(function () { test() }, 1000);