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<script src="//cdnjs.cloudflare.com/ajax/libs/moment.js/2.2.1/moment.min.js"></script>

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const dateKey = "2021-03-29"
const weekNumber = moment(dateKey, 'YYYY-MM-DD').isoWeek()
    
      
      const date = moment(dateKey, 'YYYY-MM-DD')
        .clone()
        .week(weekNumber)
        .day('Monday')
        
        console.log('date', date.format('YYYY-MM-DD'))

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Use pre-released features

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