How to Add Loan Amortization to Your App in 5 Minutes (No Math Required)

#javascript #fintech #api #webdev

Every finance app eventually needs loan amortization. The math behind it isn't hard, but implementing it correctly — handling rounding, edge cases, irregular payments — takes longer than you'd think.

Here's how to skip all that with a single API call.

The Setup

You'll need a free API key from RapidAPI. Sign up takes 30 seconds — free tier gives you 50 calls/day.

The Code

async function getAmortizationSchedule({ principal, annualRate, termMonths }) {
  const url = new URL('https://fincalcapi.p.rapidapi.com/amortize');
  url.searchParams.set('principal', principal);
  url.searchParams.set('annual_rate', annualRate);
  url.searchParams.set('term_months', termMonths);

  const response = await fetch(url, {
    headers: {
      'X-RapidAPI-Key': process.env.RAPIDAPI_KEY,
      'X-RapidAPI-Host': 'fincalcapi.p.rapidapi.com',
    },
  });

  if (!response.ok) throw new Error('API error: ' + response.status);
  return response.json();
}

const schedule = await getAmortizationSchedule({
  principal: 150000,
  annualRate: 6.0,
  termMonths: 180,
});

console.log('Monthly payment: $' + schedule.monthly_payment);
console.log('Total interest paid: $' + schedule.total_interest);

Why an API Instead of a Library?

A library (e.g., financial-js):

Adds to your bundle size
You maintain it
Edge cases require custom code

An API:

Zero bundle impact
Always up-to-date
7 other financial calculations with the same key (NPV, IRR, mortgage, ROI, etc.)

All Available Endpoints

Endpoint	What it calculates
`/amortize`	Full payment schedule
`/mortgage`	Monthly payment + total cost
`/compound-interest`	Final value with compounding
`/roi`	Simple and annualized return
`/npv`	Net present value
`/irr`	Internal rate of return
`/break-even`	Units needed to break even
`/depreciation`	Straight-line + declining balance