Ternary operator

export function isLeap(year) {
  return year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;
}

A conditional operator, also known as a "ternary conditional operator", or just "ternary operator", uses a maximum of two checks to determine if a year is a leap year.

It starts by testing the outlier condition of the year being evenly divisible by 100. It does this by using the remainder operator. If the year is evenly divisible by 100, then the expression is true, and the ternary operator returns if the year is evenly divisible by 400. If the year is not evenly divisible by 100, then the expression is false, and the ternary operator returns if the year is evenly divisible by 4.

Although it uses a maximum of only two checks, the ternary operator tests an outlier condition first, making it less efficient than another approach that would first test if the year is evenly divisible by 4, which is more likely than the year being evenly divisible by 100.

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) {
  !(year % 100) ? !(year % 400) : !(year % 4);
}

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

When the body of a function is a single expression, the function can be implemented as an arrow function, like so

export const isLeap = (year) =>
  year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;

or

export const isLeap = (year) => (!(year % 100) ? !(year % 400) : !(year % 4));

Notice that return and the curly braces are not needed.