JavaScript Get week number.

Get week number

Date.prototype.getWeek = function () {
    var onejan = new Date(this.getFullYear(), 0, 1);
    return Math.ceil((((this - onejan) / 86400000) + onejan.getDay() + 1) / 7);
};

var myDate = new Date("2001-02-02");
myDate.getWeek(); //=> 5

function getNumberOfWeek() {
    const today = new Date();
    const firstDayOfYear = new Date(today.getFullYear(), 0, 1);
    const pastDaysOfYear = (today - firstDayOfYear) / 86400000;
    return Math.ceil((pastDaysOfYear + firstDayOfYear.getDay() + 1) / 7);
}

Thanks for this 👍

Thanks for the function.

There is just small issue that typescript don't validate arithmetic operations on Date type so shows below error.

The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type

So I've called .valueOf() on Dates in that arithmetic operation.

getNumberOfWeek(): number {
    const today = new Date();
    const firstDayOfYear = new Date(today.getFullYear(), 0, 1);
    const pastDaysOfYear = (today.valueOf() - firstDayOfYear.valueOf()) / 86400000;
    return Math.ceil((pastDaysOfYear + firstDayOfYear.getDay() + 1) / 7);
}

If you want the ISO solution there is a formula here that gives you the # of weeks of a given year:

https://en.wikipedia.org/wiki/ISO_week_date

What will be week for Date ...

2022-01-01, //getDay() returning 6
2022-01-02, //getDay() returning 0
2022-01-03 //getDay() returning 1

As per above method

2022-01-01 is returning week 2 and next 6 dates is part of same week.