функция, которая принимает первым аргументов начало диапазона, а вторым - конец

const getLeapYears = (from, to) => {
  const result = [],
    isLeapYear = y => ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0);
 
  for (let year = from; year <= to; year++) {
    if (isLeapYear(year)) result.push(year);
  }
  
  return result;
}
 
console.log(getLeapYears(2000, 2018)); // [2000, 2004, 2008, 2012, 2016]

const getLeapYears = (from, to) => {
  const result = [],
    isLeapYear = function(y) {
      return ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0);
    };
 
  for (let year = from; year <= to; year++) {
    if (isLeapYear(year)) result.push(year);
  }
 
  return result;
}
 
console.log(getLeapYears(2000, 2018)); // [2000, 2004, 2008, 2012, 2016]

const getLeapYears = function(from, to) {
  const result = [],
    isLeapYear = function(y) {
      return ((y % 4 == 0) && (y % 100 != 0)) || (y % 400 == 0);
    };
 
  for (let year = from; year <= to; year++) {
    if (isLeapYear(year)) result.push(year);
  }
 
  return result;
}
 
console.log(getLeapYears(2000, 2018)); // [2000, 2004, 2008, 2012, 2016]

const getLeapYears = function(from, to) {
  const result = [],
    isLeapYear = function(y) {
      return new Date(y, 1, 29).getMonth() == 1
    }
 
  for (let year = from; year <= to; year++) {
    if (isLeapYear(year)) result.push(year);
  }
 
  return result;
}
 
console.log(getLeapYears(2000, 2018)); // [2000, 2004, 2008, 2012, 2016]