JavaScript Calculate the Day of the Year (1 - 366)

JavaScript calculate the day of the year (1 - 366)

Following OP's edit:

var now = new Date();var start = new Date(now.getFullYear(), 0, 0);var diff = now - start;var oneDay = 1000 * 60 * 60 * 24;var day = Math.floor(diff / oneDay);console.log('Day of year: ' + day);

Get full date from day of the year

The shortest way:

new Date(2016, 0, 208)

will return Date 2016-07-26

How to get the date from x-th day of the year?

Here is a function if you pass the year and the nth day of that year this will return the date of that nth day.

function dateFromDay(year, day){  var date = new Date(year, 0);   return new Date(date.setDate(day));}console.log(dateFromDay(2010, 301));console.log(dateFromDay(2010, 365));

What is the best way to determine the number of days in a month with JavaScript?

function daysInMonth (month, year) { // Use 1 for January, 2 for February, etc.
  return new Date(year, month, 0).getDate();
}

console.log(daysInMonth(2, 1999)); // February in a non-leap year.
console.log(daysInMonth(2, 2000)); // February in a leap year.

Calculate total number of days in a year

You can find the number of days using the simple leap year algorithem:

In the Gregorian calendar three criteria must be taken into account to
identify leap years: The year can be evenly divided by 4; If the year
can be evenly divided by 100, it is NOT a leap year, unless; The year
is also evenly divisible by 400. Then it is a leap year.

function daysInYear(year) {    return ((year % 4 === 0 && year % 100 > 0) || year %400 == 0) ? 366 : 365;}
console.log(daysInYear(2016));
console.log(daysInYear(2017));

How can I calculate the number of years between two dates?

Probably not the answer you're looking for, but at 2.6kb, I would not try to reinvent the wheel and I'd use something like moment.js. Does not have any dependencies.

The diff method is probably what you want: http://momentjs.com/docs/#/displaying/difference/

is there any simplest way to calculate a recurring day?

These are powers of two, meant to be used in a bit field.

You are meant to tell whether a given date is "checked" using bitwise operator &.

const SUNDAY   = 64; // 0100 0000
const SATURDAY = 32; // 0010 0000
const FRIDAY   = 16; // 0001 0000
const THURSDAY =  8; // 0000 1000
// etc.

let day = 48; // 0011 0000

day & SUNDAY   // 0 indicating Sunday should not be checked
day & SATURDAY // 32 indicating Saturday should be checked
day & FRIDAY   // 16 indicated Friday should be checked
day & THURSDAY // 0 indicated Thursday should not be checked

If you want to turn this into an array of checked values, the simplest option would be to stop using a bitfield an instead store a JSON array of strings.

Assuming that's not possible, your case statement can be replaced with something as simple as...

const DAYS = [ 1, 2, 4, 8, 16, 32, 64 ];

const dayBitfield = 41;

DAYS.filter(day => dayBitfield & day) // [1, 8, 32]

how can I convert day of year to date in javascript?

"I want to take a day of the year and convert to an actual date using the Date object."

After re-reading your question, it sounds like you have a year number, and an arbitrary day number (e.g. a number within 0..365 (or 366 for a leap year)), and you want to get a date from that.

For example:

dateFromDay(2010, 301); // "Thu Oct 28 2010", today ;)
dateFromDay(2010, 365); // "Fri Dec 31 2010"

If it's that, can be done easily:

function dateFromDay(year, day){
  var date = new Date(year, 0); // initialize a date in `year-01-01`
  return new Date(date.setDate(day)); // add the number of days
}

You could add also some validation, to ensure that the day number is withing the range of days in the year supplied.

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