Run results for: days in month

Run details:

User agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/54.0.2840.99 Safari/537.36

Browser: Chrome 54

Operating system: Windows 7

Device Platform: Other

Date tested: 7 years ago

Test name	Executions per second
manual	25992.6 Ops/sec
withDate	8130.3 Ops/sec

Script Preparation code:

function manual(date) {
  const year = date.getFullYear();
  const month = date.getMonth();
  const isLeapYear = ((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0);

  return [31, (isLeapYear ? 29 : 28), 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][month];
}

function withDate(date){
    var d= new Date(date.getFullYear(), date.getMonth()+1, 0);
    return d.getDate();
}

var date = new Date(2000, 1, 1);

Tests:

manual
for(var i=0; i<100; ++i) { manual(date); }
withDate
for(var i=0; i<100; ++i) { withDate(date); }