Payment amount function for the equivalent PMT() formula seen in Google Sheets and Excel.

payment_amount.js

/**
 * Payment amount formula as seen in Google Sheets/Excel PMT()
 * @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} pval - present value
 * @param {Number} [fv=0] - future value (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 pmt(rate, nper, pval, fv = 0, type = 0) {
    if (rate === 0) return -(pval + fv) / nper;
    if (nper === 0) throw new Error('Number of periods cannot be 0');
    /** @type {Number} */
    const pvFactor = Math.pow(1 + rate, nper);
    /** @type {Number} */
    let pmt = -rate * (pval * pvFactor + fv) / (pvFactor - 1);
    return type === 0 ? pmt : type === 1 ? pmt / (1 + rate) : 0;
}