How to convert a week number to a date in Javascript

I had tried to find some work done but I haven't had luck. Any ideas?

Examples:

Week, 1, 2001 =>开发者_高级运维 2001-01-01

Week, 26, 2007 => 2007-06-01

As Kevin's code does not implement ISO 8601 properly (first day of the first week of year must be a Monday), I've corrected it and ended up with (also check it on 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.");

I included test cases for some dates to check that the first day of the first week of year is a Monday and checked some dates based on http://www.epochconverter.com/date-and-time/weeknumbers-by-year.php

Someone might be still interested in a more contained version:

function firstDayOfWeek (year, week) {

    // Jan 1 of 'year'
    var d = new Date(year, 0, 1),
        offset = d.getTimezoneOffset();

    // ISO: week 1 is the one with the year's first Thursday 
    // so nearest Thursday: current date + 4 - current day number
    // Sunday is converted from 0 to 7
    d.setDate(d.getDate() + 4 - (d.getDay() || 7));

    // 7 days * (week - overlapping first week)
    d.setTime(d.getTime() + 7 * 24 * 60 * 60 * 1000 
        * (week + (year == d.getFullYear() ? -1 : 0 )));

    // daylight savings fix
    d.setTime(d.getTime() 
        + (d.getTimezoneOffset() - offset) * 60 * 1000);

    // back to Monday (from Thursday)
    d.setDate(d.getDate() - 3);

    return d;
}

I took the original idea from Kevin, with some tweaks, coz the original code is returning milliseconds. Here you go:

var d = firstDayOfWeek(9, 2013);

console.log(d.format("yyyy-MM-dd"));

////////////////////////////// Main Code //////////////////////////////
function firstDayOfWeek(week, year) {

    if (typeof year !== 'undefined') {
        year = (new Date()).getFullYear();
    }

    var date       = firstWeekOfYear(year),
        weekTime   = weeksToMilliseconds(week),
        targetTime = date.getTime() + weekTime - 86400000;

    var result = new Date(targetTime)

    return result; 
}

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;
}

function firstWeekday(date) {

    var day = date.getDay(),
        day = (day === 0) ? 7 : day;

    if (day > 3) {

        var remaining = 8 - day,
            target    = remaining + 1;

        date.setDate(target);
    }

    return date;
}

Take a look at this fiddle. First, it gets the first week of the specified year. This takes into account that according to ISO 8601 the first week of the year is the first week containing a wednesday. Then it adds the weeks to the acquired date and returns the result.

function firstDayOfWeek(week, year) {

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

    return date.setTime(targetTime);

}