Check if a year is a leap year in JavaScript

A leap year is a year that is evenly divisible by 4, except for end-of-century years, which must be divisible by 400.

While detecting a leap year via code sounds like a lot of work, there's a method that doesn't really involve any math whatsoever. Using the Date object, you can easily check if a given year is a leap year.

All you have to do is use the Date() constructor to create an object for February 29th of the given year. If the month is February, then Date.prototye.getMonth() will return 1 (since months are zero-based in JavaScript), meaning it's a leap year. If the month is not February, the date will be set to March 1st, which means it's not a leap year.

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

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