amortization formula in JavaScript

amortization-formula.js

// P: principal (loan amount)
// r: monthly interest rate
// M: monthly payment
// n: number of months

const P = 10000; // example loan amount
const r = 0.015; // example monthly interest rate (18% APR / 12 months)
const M = 100; // example monthly payment amount

let n = Math.log(M / (M - r * P)) / Math.log(1 + r); // calculate number of months

n = Math.ceil(n); // round up to the nearest whole number

console.log(n); // output: the number of months to pay off the loan with fixed monthly payments