The error might come from applying interest before a deposit instead of after it. It might come from treating APY as a nominal interest rate. Neither mistake necessarily causes a JavaScript error.

I run PennyRatio, a collection of personal finance calculators. This article uses a standalone savings example to explain how to test what a calculation actually means.

Define the model before testing it

Our example assumes:

• A constant annual percentage yield, or APY.
• Fixed monthly contributions.
• Contributions at the end of each month.
• Interest retained in the account.
• No withdrawals, fees, or taxes.
• Equivalent monthly growth rather than bank-specific daily accrual.

These assumptions are part of the calculation contract.

A small projection function

APY includes the effect of compounding. To preserve that annual yield, convert it into an equivalent monthly growth rate.

Save this as savings.mjs:

export function projectSavings({
  initialBalance,
  monthlyDeposit,
  apyPercent,
  months,
}) {
  const amounts = [
    initialBalance,
    monthlyDeposit,
    apyPercent,
  ];

  if (
    !amounts.every(Number.isFinite) ||
    amounts.some(value => value < 0)
  ) {
    throw new RangeError(
      "Amounts and APY must be finite, non-negative numbers."
    );
  }

  if (
    !Number.isInteger(months) ||
    months < 0 ||
    months > 1200
  ) {
    throw new RangeError(
      "Months must be an integer from 0 to 1200."
    );
  }

  const monthlyRate =
    (1 + apyPercent / 100) ** (1 / 12) - 1;

  let balance = initialBalance;

  for (let month = 0; month < months; month++) {
    balance *= 1 + monthlyRate;
    balance += monthlyDeposit;
  }

  const contributions =
    initialBalance + monthlyDeposit * months;

  if (
    !Number.isFinite(balance) ||
    !Number.isFinite(contributions)
  ) {
    throw new RangeError("Projection exceeds numeric limits.");
  }

  return {
    balance,
    contributions,
    interest: balance - contributions,
  };
}

The loop applies growth before adding each deposit. That is what makes these month-end deposits.

The 1,200-month limit is a practical boundary for this example, not a financial rule.

Test the financial meaning

Save the following as savings.test.mjs in the same folder.

import test from "node:test";
import assert from "node:assert/strict";
import { projectSavings } from "./savings.mjs";

const base = {
  initialBalance: 5000,
  monthlyDeposit: 200,
  apyPercent: 4,
  months: 12,
};

function close(actual, expected, tolerance = 1e-8) {
  assert.ok(Number.isFinite(actual));

  assert.ok(
    Math.abs(actual - expected) <= tolerance,
    `Expected ${expected}, received ${actual}`
  );
}

test("zero APY produces only contribution growth", () => {
  const result = projectSavings({
    ...base,
    apyPercent: 0,
  });

  assert.equal(result.balance, 7400);
  assert.equal(result.interest, 0);
});

test("one year preserves the stated APY", () => {
  const result = projectSavings({
    ...base,
    initialBalance: 1000,
    monthlyDeposit: 0,
  });

  close(result.balance, 1040);
});

test("month-end deposits earn no first-month interest", () => {
  const result = projectSavings({
    ...base,
    initialBalance: 0,
    months: 1,
  });

  close(result.balance, 200);
});

test("zero months produce no activity", () => {
  const result = projectSavings({
    ...base,
    months: 0,
  });

  assert.equal(result.balance, 5000);
  assert.equal(result.contributions, 5000);
  assert.equal(result.interest, 0);
});

test("matches the worked example to half a cent", () => {
  const result = projectSavings(base);

  close(result.balance, 7643.69, 0.005);
  assert.equal(result.contributions, 7400);
  close(result.interest, 243.69, 0.005);
});

test("rejects inputs outside the contract", () => {
  const invalidInputs = [
    { initialBalance: -1 },
    { monthlyDeposit: NaN },
    { apyPercent: Infinity },
    { initialBalance: "5000" },
    { months: 1.5 },
    { months: 1201 },
  ];

  for (const change of invalidInputs) {
    assert.throws(
      () => projectSavings({ ...base, ...change }),
      RangeError
    );
  }
});

Run the tests with Node’s built-in test runner:

node --test savings.test.mjs

What these tests catch

Zero APY checks that deposits alone determine growth when no interest is earned. It can also reveal division-by-zero problems in alternative implementations.

One year without deposits checks that the rate conversion preserves APY. Dividing APY by 12 and compounding it monthly would fail this check.

One month with a zero starting balance checks deposit timing. A deposit made at month-end should not earn interest during that first month.

Zero months catches loops that accidentally run one extra iteration.

The worked example checks a complete scenario:

The rate is illustrative, not a current bank offer.

Keep rounding at the display boundary

For this projection, keep intermediate values unrounded and format the result when displaying it:

const usd = new Intl.NumberFormat("en-US", {
  style: "currency",
  currency: "USD",
});

const result = projectSavings(base);

console.log(usd.format(result.balance));
// "$7,643.69"

Repeatedly rounding each month changes the model. Also, toFixed() returns a string.

If you need to reproduce a specific institution’s calculation, its accrual and rounding rules must be modeled explicitly.

Payment processing and ledger accounting also require a suitable monetary representation and a defined rounding policy. This floating-point example is intended for educational projections.

Test the interface separately

Correct calculation code does not guarantee correct user input.

For example:

Number(""); // 0

A blank required field should produce a helpful message instead of silently becoming zero.

Check that your interface:

• Distinguishes missing inputs from valid zero values.
• Labels APY clearly.
• States when deposits occur.
• Separates contributions from estimated interest.
• Keeps important assumptions readable on mobile.

Make limitations visible

Place a short explanation beside the result:

Assumes constant APY and month-end deposits. Excludes fees, taxes, and withdrawals. Actual earnings may differ.

Real account rates and deposit dates can change. Banks may use different daily accrual and interest-crediting practices.

For the official APY framework, see the CFPB’s Appendix A to Regulation DD.

You can also explore savings scenarios with the PennyRatio HYSA Calculator.

Disclosure: PennyRatio is my project. The code in this article is a separate teaching example, not a reproduction of