addBusinessDays.js

[...Array(100).keys()].forEach((i) => {
  try {
    const startDate = new Date()
    // Adjust here if you want to stage a different date
    // startDate.setDate(startDate.getDate() + 3) // 3 days in the future
    addBusinessDays(startDate, i, true)
  } catch (err) {
    console.error(err.message)
  }
})

function addBusinessDays(startDate, dplus, adjustSilently = false) {
  const verbose = true // Enable extra diagnostic output
  const d = new Date(startDate.getTime())

  // If the incoming date is in a weekend, push to the next weekday
  // adjustSilently governs display (or not) of forced date change
  if (d.getDay() === 0) {
    if (!adjustSilently) { // Display forced input date change?
      console.error(`Error: invalid day of week: ${d.getDay()}; expected value between 1 and 5, inclusive. Forcing to next business day`)
    }
    d.setDate(d.getDate() + 1)
    if (!adjustSilently) { console.log(`new d: ${d.getMonth()+1}/${d.getDate()}/${d.getFullYear()}`) }
  } else if (d.getDay() === 6) {
    if (!adjustSilently) { // Display forced input date change?
      console.error(`Error: invalid day of week: ${d.getDay()}; expected value between 1 and 5, inclusive. Forcing to next business day`)
    }
    d.setDate(d.getDate() + 2)
    if (!adjustSilently) { console.log(`new d: ${d.getMonth()+1}/${d.getDate()}/${d.getFullYear()}`) }
  }

  const now = new Date(d.getTime())

  const d2we = 6 - d.getDay()
  const d2m  = d2we + 2
  let daystoadd = dplus
  
  if (dplus >= d2we) {
    daystoadd = (d2m - 1) + (dplus - (d2we - 1)) + (Math.floor((dplus-d2we)/5) * 2)
  }

  d.setDate(d.getDate() + daystoadd)

  if (verbose) {
    console.log(`${now.getMonth()+1}/${now.getDate()}/${now.getFullYear()} + ${dplus} = ${d.getMonth()+1}/${d.getDate()}/${d.getFullYear()}`)
  }

  // If the newly calculated date is on a weekend, throw a new Error
  if (d.getDay() === 0 || d.getDay() === 6) {
    throw new Error(`Error: invalid result day of week: ${d.getDay()}; Value must be between 1 and 5, inclusive.`)
  }

  return(d)
}