Comment convertir un numéro de semaine à une date en JavaScript

Comme le code de Kevin n'entraîne pas correctement ISO 8601 (le premier jour de la première semaine de la semaine doit être un lundi), je l'ai corrigé et fini par (aussi Vérifiez-le sur jsfiddle ):

function firstDayOfWeek(week, year) { 

    if (year==null) {
        year = (new Date()).getFullYear();
    }

    var date       = firstWeekOfYear(year),
        weekTime   = weeksToMilliseconds(week),
        targetTime = date.getTime() + weekTime;

    return date.setTime(targetTime); 

}

function weeksToMilliseconds(weeks) {
    return 1000 * 60 * 60 * 24 * 7 * (weeks - 1);
}

function firstWeekOfYear(year) {
    var date = new Date();
    date = firstDayOfYear(date,year);
    date = firstWeekday(date);
    return date;
}

function firstDayOfYear(date, year) {
    date.setYear(year);
    date.setDate(1);
    date.setMonth(0);
    date.setHours(0);
    date.setMinutes(0);
    date.setSeconds(0);
    date.setMilliseconds(0);
    return date;
}

/**
 * Sets the given date as the first day of week of the first week of year.
 */
function firstWeekday(firstOfJanuaryDate) {
    // 0 correspond au dimanche et 6 correspond au samedi.
    var FIRST_DAY_OF_WEEK = 1; // Monday, according to iso8601
    var WEEK_LENGTH = 7; // 7 days per week
    var day = firstOfJanuaryDate.getDay();
    day = (day === 0) ? 7 : day; // make the days monday-sunday equals to 1-7 instead of 0-6
    var dayOffset=-day+FIRST_DAY_OF_WEEK; // dayOffset will correct the date in order to get a Monday
    if (WEEK_LENGTH-day+1<4) {
        // the current week has not the minimum 4 days required by iso 8601 => add one week
        dayOffset += WEEK_LENGTH;
    }
    return new Date(firstOfJanuaryDate.getTime()+dayOffset*24*60*60*1000);
}

function assertDateEquals(effectiveDate, expectedDate, description) {
    if ((effectiveDate==null ^ expectedDate==null) || effectiveDate.getTime()!=expectedDate.getTime()) {
        console.log("assert failed: "+description+"; effective="+effectiveDate+", expected="+expectedDate);
    }
}
function assertEquals(effectiveValue, expectedValue, description) {
    if (effectiveValue!=expectedValue) {
        console.log("assert failed: "+description+"; effective="+effectiveValue+", expected="+expectedValue);
    }
}

// expect the first day of year to be a monday
for (var i=1970; i<2050; i++) {
    assertEquals(firstWeekOfYear(i).getDay(), 1, "first day of year "+i+" must be a monday"); // 1=Monday
}

// assert some future first day of first week of year; source: http://www.epochconverter.com/date-and-time/weeknumbers-by-year.php
assertDateEquals(firstWeekOfYear(2013), new Date(Date.parse("Dec 31, 2012")), "2013");
assertDateEquals(firstWeekOfYear(2014), new Date(Date.parse("Dec 30, 2013")), "2014");
assertDateEquals(firstWeekOfYear(2015), new Date(Date.parse("Dec 29, 2014")), "2015");
assertDateEquals(firstWeekOfYear(2016), new Date(Date.parse("Jan 4, 2016")), "2016");
assertDateEquals(firstWeekOfYear(2017), new Date(Date.parse("Jan 2, 2017")), "2017");
assertDateEquals(firstWeekOfYear(2018), new Date(Date.parse("Jan 1, 2018")), "2018");
assertDateEquals(firstWeekOfYear(2019), new Date(Date.parse("Dec 31, 2018")), "2019");
assertDateEquals(firstWeekOfYear(2020), new Date(Date.parse("Dec 30, 2019")), "2020");
assertDateEquals(firstWeekOfYear(2021), new Date(Date.parse("Jan 4, 2021")), "2021");
assertDateEquals(firstWeekOfYear(2022), new Date(Date.parse("Jan 3, 2022")), "2022");
assertDateEquals(firstWeekOfYear(2023), new Date(Date.parse("Jan 2, 2023")), "2023");
assertDateEquals(firstWeekOfYear(2024), new Date(Date.parse("Jan 1, 2024")), "2024");
assertDateEquals(firstWeekOfYear(2025), new Date(Date.parse("Dec 30, 2024")), "2025");
assertDateEquals(firstWeekOfYear(2026), new Date(Date.parse("Dec 29, 2025")), "2026");

console.log("All assertions done.");

Quelqu'un peut toujours être intéressé par une version plus contenue: xxx