Problems › JavaScript › dates
How many days in this month
A date picker greys out the days that do not exist in the month being shown.
- Months are numbered 1 to 12.
- February has 29 days in a leap year and 28 otherwise; the leap rule is the usual 4 / 100 / 400 one.
- A month outside 1 to 12 gives 0.
daysInMonth(year: int, month: int) → int
Where you start
function daysInMonth(year, month) {
}Worked examples
| Call | Result |
|---|---|
daysInMonth(2024, 2) | 29 |
daysInMonth(2023, 2) | 28 |
daysInMonth(1900, 2) | 28 |
daysInMonth(2024, 4) | 30 |
Hint
A lookup table for the eleven easy months, and one branch for February.
Reference solution in JavaScript
function daysInMonth(year, month) {
if (month < 1 || month > 12) return 0;
const table = [31, 0, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
if (month !== 2) return table[month - 1];
const leap = year % 400 === 0 || (year % 4 === 0 && year % 100 !== 0);
return leap ? 29 : 28;
}