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How to Calculate Date from Week Number in JavaScript?

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To calculate date from week number in JavaScript, we can use the JavaScript date setDate and getDate methods.

For instance, we can write:

const getSundayFromWeekNum = (weekNum, year) => {
  const sunday = new Date(year, 0, (1 + (weekNum - 1) * 7));
  while (sunday.getDay() !== 0) {
    sunday.setDate(sunday.getDate() - 1);
  }
  return sunday;
}

console.log(getSundayFromWeekNum(10, 2021))

We create the getSundayFromWeekNum function that takes the weekNum and year parameters that we use to get the Sunday of the given year and week number.

In the function, we create a JavaScript date with the Date constructor with year, 0, (1 + (weekNum - 1) * 7) to get the date given weekNum and year.

Then we add a while loop to subtract 1 day from the sundday date until we reach a Sunday.

Finally, we return that result.

Then when we console log, we get Sun Feb 28 2021 00:00:00 GMT-0800 (Pacific Standard Time).

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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