Through the 'date-fns' module, I am receiving numbers of how many weeks the date is this year.

const current = '2022-03-10'
const weekNumber = getWeek(current, 1) // 11

On the contrary, if you only know the numbers, you want to know how to do the first date of the week number.

The way I want to know.

const weekNumber = 11;
const weekOfstartDate = anyFunc(weekNumber) // '2022-03-07'

Do you know the answer to this solution?

Through the 'date-fns' module, I am receiving numbers of how many weeks the date is this year.

const current = '2022-03-10'
const weekNumber = getWeek(current, 1) // 11

On the contrary, if you only know the numbers, you want to know how to do the first date of the week number.

The way I want to know.

const weekNumber = 11;
const weekOfstartDate = anyFunc(weekNumber) // '2022-03-07'

Do you know the answer to this solution?

• javascript
• date
• date-fns

• That's simply not possible. Week 1 can correspond to 2022-01-04 or 2022-01-05 or even 1994-01-04, and you have no way to tell which it is, the information is lost. – Kaiido Commented Mar 3, 2022 at 8:24
• @Kaiido Is it possible if we limit it to 2022 years only? – Brian Commented Mar 3, 2022 at 8:31
• No, you can't tell apart the 7 days that pose the given week. – Kaiido Commented Mar 3, 2022 at 8:32
• What's your start of the week? Is it sunday or monday? – Marc Anthony B Commented Mar 3, 2022 at 8:48
• 1 @T.J.Crowder ah you think that's what they want? I was under the impression they wanted to retrieve any day in that week but you're probably right. They would also need the value of firstWeekContainsDate though, since the 1st of January 2022 was actually on week 51 of the year 2021. – Kaiido Commented Mar 3, 2022 at 8:55

4 Answers 4

You can use the I token:

var dateFns = require("date-fns");
console.log(dateFns.parse('10','I', new Date()));

At npm.runkit. that returns a date for Mon 7 Mar 2022. It seems date-fns assumes the year of the current week. I have no idea how to specify the year, attempting:

console.log(dateFns.parse('2022-10','YYYY-II', new Date(),{
  useAdditionalWeekYearTokens: true
}));

Throws a range error: The format string mustn't contain YYYY and II at the same time. Similarly for "2022W10" and tokens "YYYY'W'II", it says Y and I tokens can't be in the same format string.

A function to do the same thing is:

// Returns the first day (Monday) of the specified week
// Year defaults to the current local calendar year
function getISOWeek(w, y = new Date().getFullYear()) {
  let d = new Date(y, 0, 4);
  d.setDate(d.getDate() - (d.getDay() || 7) + 1 + 7*(w - 1));
  return d;
}

// Mon Mar 14 2022
console.log(getISOWeek(11).toString());
// Mon Jan 02 2023
console.log(getISOWeek(1,2023).toString());
// Mon Dec 28 2026
console.log(getISOWeek(53,2026).toString());

A robust function should validate the input such that the week number is 1 to 53 and that 53 is only permitted for years that have 53 weeks.

I wanted a solution only using date-fns functions. This is how i bined it:

const result = setWeek(nextMonday(new Date(year, 0, 4)), week, {
    weekStartsOn: 1,
    firstWeekContainsDate: 4,
  });

This is how to specify year for date-fns it uses date provided gather relevant information such as year. So setting year on relative date for parse tells date-fns which year that 10th week is occuring in.

console.log(dateFns.parse('10','I', new Date().setYear(2002)));

const weekNumber = 11;
const year = 2022
const weekOfstartDate = dateFns.parse(weekNumber.toString(), "I", new Date().setFullYear(year));  // '2022-03-14'

Note that the result is 2022-03-14 but not 2022-03-07 because the 14th week of 2022 starts from 14.03