Boolean Chaining

function! LeapYear(year) abort
  return a:year % 4 == 0 && a:year % 100 != 0 || a:year % 400 == 0
endfunction

In the snippet year % 4 == 0, year is first divided by 4, returning the remainder or portion left over. If the year is evenly divisible by 4, the remainder would be 0 and 0 == 0 produces true. This check will be repeated two more times for our other rules. The logical AND (&&) and OR (||) operators are used to produce a single value from the expressions on either side. These operators notably short-circuit and only evaluate the right-hand side in the following situations. The && operator only evaluates and returns the right-hand side when the left-hand side is truthy. If the left-hand side is falsey, it is returned without evaluating the right-hand side. The || operator only evaluates and returns the right-hand side when the left-hand side is falsey. If the left-hand side is truthy, it is returned without evaluating the right-hand side. In the below table, Vim script represents true and false as 1 and 0.