To get a list of all days in a month, you can start with a Date on the first day of a month, increase the day until the month changes.
/**
* @param {int} The month number, 0 based
* @param {int} The year, not zero based, required to account for leap years
* @return {Date[]} List with date objects for each day of the month
*/
function getDaysInMonth(month, year) {
var date = new Date(year, month, 1);
var days = [];
while (date.getMonth() === month) {
days.push(new Date(date));
date.setDate(date.getDate() + 1);
}
return days;
}
UTC Version
In response to some comments, I’ve created a version that uses UTC methods in case you want to call UTC methods instead of the standard methods that return the localized time zone.
I suspect this is the culprit of the comments saying this didn’t work. You typically want to make sure you call getUTCMonth/Day/Hours methods if you instantiated it with Date.UTC, and vice-versa, unless you are trying to convert time zones and show differences.
function getDaysInMonthUTC(month, year) {
var date = new Date(Date.UTC(year, month, 1));
var days = [];
while (date.getUTCMonth() === month) {
days.push(new Date(date));
date.setUTCDate(date.getUTCDate() + 1);
}
return days;
}
Editing This Answer
If you think there’s a problem with this script, please feel free to:
- First see existing unit tests below
- Write a test case that proves it’s broken.
- Fix the code, making sure existing tests pass.
Unit Tests
/**
* @param {int} The month number, 0 based
* @param {int} The year, not zero based, required to account for leap years
* @return {Date[]} List with date objects for each day of the month
*/
function getDaysInMonthUTC(month, year) {
var date = new Date(Date.UTC(year, month, 1));
var days = [];
while (date.getUTCMonth() === month) {
days.push(new Date(date));
date.setUTCDate(date.getUTCDate() + 1);
}
return days;
}
function getDaysInMonth(month, year) {
var date = new Date(year, month, 1);
var days = [];
while (date.getMonth() === month) {
days.push(new Date(date));
date.setDate(date.getDate() + 1);
}
return days;
}
const days2020 = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
const days2021 = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
describe("getDaysInMonthUTC", function() {
it("gets day counts for leap years", function() {
const actual = days2020.map(
(day, index) => getDaysInMonthUTC(index, 2020).length
);
expect(actual).toEqual(days2020);
});
it("gets day counts for non-leap years", function() {
const actual = days2021.map(
(day, index) => getDaysInMonthUTC(index, 2021).length
);
expect(actual).toEqual(days2021);
});
});
describe("getDaysInMonth", function() {
it("gets day counts for leap years", function() {
const actual = days2020.map(
(day, index) => getDaysInMonth(index, 2020).length
);
expect(actual).toEqual(days2020);
});
it("gets day counts for non-leap years", function() {
const actual = days2021.map(
(day, index) => getDaysInMonth(index, 2021).length
);
expect(actual).toEqual(days2021);
});
});
// load jasmine htmlReporter
(function() {
var env = jasmine.getEnv();
env.addReporter(new jasmine.HtmlReporter());
env.execute();
}());<script src="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine.js"></script>
<script src="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine-html.js"></script>
<link href="https://cdn.jsdelivr.net/jasmine/1.3.1/jasmine.css" rel="stylesheet"/>