Bonds compound interest

bonds.js

/**
 * --- WHAT IS IT?
 *
 * This is a custom function for use in Google Sheets and allows you to count
 * the compound interest of bonds from provided date to today.
 *
 * --- HOW TO USE IT?
 *
 * 1. Create a new one or open an existing Google Sheet.
 * 2. Navigate to `Extensions => Apps Scripts` in the navigation bar.
 * 3. Paste the function into the Script editor.
 * 4. Click `Run`.
 * 5. Back to the sheet, now you can use the `BONDS` function like any other function.
 *
 * --- EXAMPLES
 *
 * =BONDS(100; '2021-05-18'; 1.7)           //18 Nov 2021 will return 100.85
 * =BONDS(100; '2020-11-18'; 1.7)           //18 Nov 2021 will return 101.70
 * =BONDS(100; '2019-11-18'; 1.7; 5.0)      //18 Nov 2021 will return 106.78
 * =BONDS(100; '2018-11-18'; 1.7; 5.0; 4.5) //18 Nov 2021 will return 111.59
 */

/**
 * Returns the bonds compound interest since date.
 *
 * @param {number} buyPrice The buying price.
 * @param {date} buyDate The buying date.
 * @param {...number} yearsInterests An infinite number of parameters with each year interest.
 *
 * @customfunction
 *
 * @author Damian Nowak <me@dnowak.dev>
 */
function BONDS(buyPrice, buyDate, ...yearsInterests) {
  const today = new Date(new Date().setHours(0, 0, 0, 0))
  const dayInSeconds = 1000 * 60 * 60 * 24
  let price = parseFloat(buyPrice)
  let date = new Date(new Date(buyDate).setHours(0, 0, 0, 0))

  if (0 >= price) {
    throw new Error('Invalid buy price')
  }

  if (false === date instanceof Date || isNaN(date) || date > today) {
    throw new Error('Invalid buy date')
  }

  let i = 0

  do {
    const datePlusYear = new Date(
      date.getFullYear() + 1,
      date.getMonth(),
      date.getDate()
    )

    const dateTo = datePlusYear >= today ? today : datePlusYear
    const numberOfDays = Math.round((datePlusYear - date) / dayInSeconds)
    const numberOfToDate = Math.round((dateTo - date) / dayInSeconds)
    const percentPerDay =
      parseFloat((yearsInterests[i++] || '1').toString().replace(',', '.')) /
      numberOfDays /
      100

    price = price + price * numberOfToDate * percentPerDay
    date = datePlusYear
  } while (date < today)

  return parseFloat(price.toFixed(2))
}