Future Value formulas as seen in Excel in Google Sheets written in Javascript format.

future_value.js

/**
 * Future Value formula as detailed in Google Sheets/Excel
 * @param {Number} rate - as a decimal not as a percentage per period
 * @param {Number} nper - total number of periods remaining, needs to be the same period as rate
 * @param {Number} [pymt=0] - amount paid per period (optional is pval is not 0, otherwise default is 0)
 * @param {Number} [pval=0] - present value of the annuity (optional if pymt is 0, otherwise default is 0)
 * @param {Number} [type=0] - is payment in arrears, paid at the end of each period, (0) or in advance, paid at the start of each period (1) (default is 0 = arrears)
 * @returns {Number}
 */
function fv(rate, nper, pymt = 0, pval = 0, type = 0) {
    if (pymt === 0 && pval === 0) return 0;
    if (pymt === 0) return -pval * Math.pow(1 + rate, nper);
    if (rate === 0) return -pval - (pymt * nper);
    if (pval !== 0 && pymt === 0) pymt -= -pmt(rate, nper, pval, 0, type);    
    
    // Calculate the future value of the initial amount
    const fvOfPVal = pval * Math.pow(1 + rate, nper);

    // Calculate the future value of the periodic payments
    let fvOfPymts = pymt * (Math.pow(1 + rate, nper) - 1) / rate;
    // Adjust for payment at the beginning of the period        
    fvOfPymts *= type === 1 ? (1 + rate) : 1;

    // The future value is the sum of the FV of the lump sum and the FV of the payments
    // Because these represent outflows, we use a negative sign
    return -(fvOfPVal + fvOfPymts);
}