make date and time constantly update javascript code example

Example: auto update time in html

timer();

function timer(){
 var currentTime = new Date()
var hours = currentTime.getHours()
var minutes = currentTime.getMinutes()
var sec = currentTime.getSeconds()
if (minutes < 10){
    minutes = "0" + minutes
}
if (sec < 10){
    sec = "0" + sec
}
var t_str = hours + ":" + minutes + ":" + sec + " ";
if(hours > 11){
    t_str += "PM";
} else {
   t_str += "AM";
}
 document.getElementById('time_span').innerHTML = t_str;
 setTimeout(timer,1000);
}