setInterval method sets up a recurring timer. It returns a handle that you can pass into clearInterval to stop it from firing: JS code:- 

var myVar = setInterval(myTimer, 1000);

function myTimer() {
  var d = new Date();
  var t = d.toLocaleTimeString();
  document.getElementById("demo").innerHTML = t;
}

function myStopFunction() {
  clearInterval(myVar);
}