moment().duration().weeks();

npm install moment

const moment = require('moment'); 
  
let durationOne = moment.duration(3, 'weeks'); 
let durationTwo = moment.duration(6, 'weeks'); 
  
// This returns 3 as it would be the 3rd week 
// in the first month of the duration 
console.log( 
  "durationOne Weeks is:", durationOne.weeks() 
) 
  
// This returns 1 as it would be the 1st week 
// in the second month of the duration 
console.log( 
  "durationTwo Weeks is:", durationTwo.weeks() 
)

durationOne Weeks is: 3
durationTwo Weeks is: 1

const moment = require('moment'); 
  
let durationA = moment.duration(30, 'days'); 
let durationB = moment.duration(38, 'days'); 
  
// The asWeeks() method will return a value 
// of the actual number of weeks of the duration 
console.log( 
  "Length of durationA in weeks is:", 
  durationA.asWeeks() 
) 
  
// The weeks() method will return  
// the week of the duration 
// It can be denoted as floor(numberOfWeeks % 4) 
console.log( 
  "durationA Weeks is:", durationA.weeks() 
) 
  
console.log( 
  "Length of durationB in weeks is:", 
  durationB.asWeeks() 
) 
console.log( 
  "durationB Weeks is:", durationB.weeks() 
)

Length of durationA in weeks is: 4.285714285714286
durationA Weeks is: 4
Length of durationB in weeks is: 5.428571428571429
durationB Weeks is: 1