Live Demo: JavaScript Program to Check Whether Leap Year or Not

<!DOCTYPE html>
<html>
  <body>
<script>
 function checkLeapYear() {
	 // fetch the input
	 let year=document.getElementById("year").value;
	  
	 //conditions to find out the leap year
	 if ((0 == year % 4) && (0 != year % 100) || (0 == year % 400)) {
		document.write(year + ' is a leap year');
	 }
	 else {
		document.write(year + ' is not a leap year');
	 }
 } 
</script>
  <form>
	 Enter year <input type="number" id="year" />
    <input type="button" value="submit" onclick="checkLeapYear()" />
  </form>
  </body>
</html>