Example of validation

HTML

First name: <input id="fname" type="text"> <input id="btn" value="check" type="button">

JavaScript

document.getElementById("btn").addEventListener("click", function (even) {
	var fnameCtrl = document.getElementById("fname");
  var fname = fnameCtrl.value;
  if(fname === "") {
  	fnameCtrl.setAttribute("style", "border: red 1px solid;").createElement("span").innerHTML("Mandatory Field");
  } else {
  	fnameCtrl.removeAttribute("style").removeElement('span');
  }
});