JavaScript one-liner function: isLeapYear (Proleptic Gregorian calendar)

isLeapYear.js

// =================================================================
// https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar
// =================================================================

// for ES2015
{
  const isLeapYear = y => !!(+y===parseInt(y)&&!y.pop&&(y%4)^(y%100)|!(y%400));

  const testParams = [
    // expect: true  (is leap)
    0, 4, 400, -4,
    "0", "4", "400", "-4",

    // expect: false (is not leap)
    1, 100, -1, 4.1,
    "1", "100", "-1", "4.1",

    // expect: false (invalid)
    "4test", {a:4}, [4], [400],
    "", true, false, null, undefined, NaN,
  ];

  for (p of testParams) {
    const t = p !== p ? p : JSON.stringify(p);
    console.log(`${t}: ${isLeapYear(p)}`);
  }
}

// for ES5
(function() {
  var isLeapYear = function(y) {
    return !!(+y===parseInt(y)&&!y.pop&&(y%4)^(y%100)|!(y%400));
  };

  var testParams = [
    // expect: true  (is leap)
    0, 4, 400, -4,
    "0", "4", "400", "-4",

    // expect: false (is not leap)
    1, 100, -1, 4.1,
    "1", "100", "-1", "4.1",

    // expect: false (invalid)
    "4test", {a:4}, [4], [400],
    "", true, false, null, undefined, NaN,
  ];

  testParams.forEach(p => {
    var t = p !== p ? p : JSON.stringify(p);
    console.log(t + ": " + isLeapYear(p));
  });
})();