switch statement

export function isLeap(year) {
  switch (true) {
    case year % 400 == 0:
      return true;
    case year % 100 == 0:
      return false;
    default:
      return year % 4 == 0;
  }
}

The switch statement tests the value true, which leaves the actual testing up to the case arms. The default arm of the switch returns whether the year is evenly divisable by 4 when none of the previous arms match.

The switch statement is somewhat more verbose than other approaches, and may also be considered less readable.

Shortening

By using the falsiness of 0, a test for a value equaling 0 can be shortened using the logical NOT operator, like so

export function isLeap(year) {
  switch (true) {
    case !(year % 400):
      return true;
    case !(year % 100):
      return false;
    default:
      return !(year % 4);
  }
}

It can be thought of as the expression not having a remainder.