Leap year Question in Javascript using nested if-else

In the nested if statement, I have given all the conditions. But some leap years are not being shown as Leap year. For eg: 2000 is coming as a leap year, but year like 2016, 2020 aren't being considered as a leap year. Please Help.

var y = prompt("Enter the year");

if (y % 4 === 0) {
  if (y % 100 === 0) {
    if (y % 400 === 0) {
      alert(y + " is a leap year");
    } else {
      alert(y + " is not a leap year");
    }
  } else {
    alert(y + " is not a leap year");
  }
} else {
  alert(y + " is not a leap year");
}