isLeapYear

Returns true if the given year is a leap year, false otherwise.

Use new Date(), setting the date to February 29th of the given year and use Date.prototype.getMonth() to check if the month is equal to 1.

const isLeapYear = (year) => new Date(year, 1, 29).getMonth() === 1;

isLeapYear(2019); // false
isLeapYear(2020); // true