Add realistic loan amortization with currentBalance back-calculation
- Extend schema with effectiveAnnualRate, totalInterest, totalAmount - Back-calculate paid months from currentBalance and rebuild schedule - Allow schedule calculation from termMonths without monthlyPayment - Handle NaN form values gracefully - Show effective rate, total interest and total amount in UI - Add amortization unit tests
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Matthias
135
convex/lib/amortization.test.ts
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135
convex/lib/amortization.test.ts
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/// <reference types="vite/client" />
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import { describe, expect, test } from "vitest";
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import { buildSchedule, computePaymentFromTerm, paymentsMade } from "./amortization";
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describe("amortization", () => {
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test("builds schedule without currentBalance from start", () => {
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const result = buildSchedule({
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principal: 10000,
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annualRate: 3.5,
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startDate: new Date("2024-01-15"),
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termMonths: 24,
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});
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expect(result.schedule).toHaveLength(24);
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expect(result.payment).toBeGreaterThan(0);
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expect(result.totalAmount).toBeGreaterThan(10000);
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expect(result.totalInterest).toBeGreaterThan(0);
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expect(result.paidMonths).toBe(0);
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expect(result.currentBalance).toBe(result.schedule[result.schedule.length - 1].balance);
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});
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test("computes monthly payment for a 60 month loan", () => {
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const payment = computePaymentFromTerm(10000, 3.5, 60);
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expect(payment).toBeCloseTo(181.92, 2);
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});
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test("calculates paid months from currentBalance", () => {
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const principal = 10000;
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const annualRate = 3.5;
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const termMonths = 60;
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const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);
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const paidMonths = paymentsMade(principal, annualRate, monthlyPayment, 8135.81);
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expect(paidMonths).toBe(12);
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});
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test("reconstructs plan from currentBalance and keeps total deviation small", () => {
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const principal = 10000;
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const annualRate = 3.5;
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const termMonths = 60;
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const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);
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const result = buildSchedule({
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principal,
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annualRate,
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startDate: new Date("2024-01-15"),
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monthlyPayment,
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currentBalance: 8135.81,
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});
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expect(result.paidMonths).toBe(12);
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expect(result.schedule[11].balance).toBeCloseTo(8135.81, 2);
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expect(result.schedule).toHaveLength(60);
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expect(result.totalAmount).toBeCloseTo(principal + result.totalInterest, 0);
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expect(result.totalAmount).toBeGreaterThanOrEqual(principal);
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});
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test("builds schedule from termMonths when monthlyPayment is omitted", () => {
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const result = buildSchedule({
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principal: 12000,
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annualRate: 4.5,
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startDate: new Date("2024-01-15"),
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termMonths: 48,
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});
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expect(result.payment).toBeGreaterThan(0);
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expect(result.schedule).toHaveLength(48);
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expect(result.totalInterest).toBeGreaterThan(0);
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});
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test("reconstructs plan from termMonths and currentBalance without monthlyPayment", () => {
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const principal = 12000;
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const annualRate = 4.5;
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const termMonths = 48;
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const result = buildSchedule({
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principal,
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annualRate,
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startDate: new Date("2024-01-15"),
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termMonths,
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currentBalance: 7634.12,
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});
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expect(result.payment).toBeGreaterThan(0);
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expect(result.paidMonths).toBeGreaterThan(0);
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expect(result.paidMonths).toBeLessThan(48);
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expect(result.schedule[result.paidMonths - 1].balance).toBeCloseTo(7634.12, 2);
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expect(result.schedule).toHaveLength(48);
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});
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test("throws when neither monthlyPayment nor termMonths is provided", () => {
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expect(() =>
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buildSchedule({
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principal: 10000,
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annualRate: 3.5,
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startDate: new Date("2024-01-15"),
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}),
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).toThrow("Entweder monthlyPayment oder termMonths erforderlich");
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});
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test("handles zero interest schedule", () => {
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const result = buildSchedule({
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principal: 12000,
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annualRate: 0,
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startDate: new Date("2024-01-15"),
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monthlyPayment: 200,
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});
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expect(result.schedule).toHaveLength(60);
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expect(result.totalInterest).toBe(0);
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expect(result.totalAmount).toBe(12000);
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});
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test("returns realistic plan for user scenario with currentBalance", () => {
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const principal = 1860;
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const annualRate = 5.99;
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const termMonths = 24;
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const monthlyPayment = computePaymentFromTerm(principal, annualRate, termMonths);
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const result = buildSchedule({
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principal,
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annualRate,
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startDate: new Date("2024-01-01"),
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monthlyPayment,
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currentBalance: 775.25,
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asOf: new Date("2025-06-01"),
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});
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expect(result.paidMonths).toBeGreaterThan(0);
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expect(result.paidMonths).toBeLessThan(24);
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expect(result.schedule[result.paidMonths - 1].balance).toBeCloseTo(775.25, 2);
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expect(result.schedule).toHaveLength(24);
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});
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});
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@@ -12,7 +12,10 @@ export type ScheduleResult = {
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payment: number;
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termMonths: number;
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totalInterest: number;
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totalAmount: number;
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payoffDate: Date;
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paidMonths: number;
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currentBalance: number;
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};
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function round2(n: number): number {
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@@ -58,18 +61,100 @@ export function computePaymentFromTerm(
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return round2(payment);
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}
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export function paymentsMade(
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principal: number,
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annualRate: number,
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monthlyPayment: number,
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currentBalance: number,
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): number {
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const i = annualRate / 100 / 12;
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const B = Math.max(0, currentBalance);
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if (Number.isNaN(B) || Number.isNaN(monthlyPayment) || B <= 0 || monthlyPayment <= 0) {
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return 0;
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}
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if (i <= 0) {
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const paidPrincipal = principal - B;
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return Math.max(0, Math.floor(paidPrincipal / monthlyPayment));
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}
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const ratio = (monthlyPayment - i * B) / (monthlyPayment - i * principal);
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if (ratio <= 0 || Number.isNaN(ratio)) {
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return 0;
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}
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const n = Math.log(ratio) / Math.log(1 + i);
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return Math.max(0, Math.floor(n));
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}
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function buildFutureSchedule(
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startingBalance: number,
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annualRate: number,
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monthlyPayment: number,
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startDate: Date,
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startMonthIndex: number,
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): AmortRow[] {
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const i = annualRate / 100 / 12;
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const schedule: AmortRow[] = [];
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let balance = round2(startingBalance);
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if (Number.isNaN(balance) || balance <= 0 || Number.isNaN(monthlyPayment) || monthlyPayment <= 0) {
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return schedule;
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}
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let month = startMonthIndex;
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while (balance > 0) {
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const interest = round2(i > 0 ? balance * i : 0);
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let principalPart = round2(monthlyPayment - interest);
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let payment = monthlyPayment;
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if (principalPart >= balance) {
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principalPart = round2(balance);
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payment = round2(interest + principalPart);
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}
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balance = round2(balance - principalPart);
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if (balance < 0.01) {
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balance = 0;
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}
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schedule.push({
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month,
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date: formatDate(addMonths(startDate, month - 1)),
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payment,
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interest,
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principal: principalPart,
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balance: Math.max(0, balance),
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});
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if (balance <= 0) break;
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month++;
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if (month > startMonthIndex + 1200) {
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throw new Error("Tilgungsplan kann nicht berechnet werden: Laufzeit zu lang");
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}
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}
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return schedule;
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}
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export function buildSchedule(input: {
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principal: number;
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annualRate: number;
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startDate: Date;
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monthlyPayment?: number;
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termMonths?: number;
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currentBalance?: number;
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asOf?: Date;
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}): ScheduleResult {
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const P = input.principal;
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const i = input.annualRate / 100 / 12;
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let payment = input.monthlyPayment;
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let termMonths = input.termMonths;
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let payment = Number.isNaN(input.monthlyPayment ?? 0) ? undefined : input.monthlyPayment;
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let termMonths = Number.isNaN(input.termMonths ?? 0) ? undefined : input.termMonths;
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if (payment !== undefined && termMonths === undefined) {
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termMonths = computeTermFromPayment(P, input.annualRate, payment);
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@@ -85,19 +170,66 @@ export function buildSchedule(input: {
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const schedule: AmortRow[] = [];
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let balance = P;
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for (let month = 1; month <= termMonths; month++) {
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const currentBalance = Number.isNaN(input.currentBalance ?? 0) ? undefined : input.currentBalance;
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let paidMonths = 0;
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if (currentBalance !== undefined) {
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paidMonths = paymentsMade(P, input.annualRate, payment, currentBalance);
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}
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if (currentBalance === undefined || paidMonths === 0) {
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for (let month = 1; month <= termMonths; month++) {
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const interest = round2(i > 0 ? balance * i : 0);
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let principalPart = round2(payment - interest);
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let actualPayment = payment;
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if (month === termMonths || principalPart > balance) {
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principalPart = round2(balance);
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actualPayment = round2(interest + principalPart);
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}
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balance = round2(balance - principalPart);
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const date = formatDate(addMonths(input.startDate, month - 1));
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schedule.push({
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month,
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date,
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payment: actualPayment,
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interest,
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principal: principalPart,
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balance: Math.max(0, balance),
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});
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if (balance <= 0) break;
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}
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const actualTerm = schedule.length;
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const totalPaid = schedule.reduce((sum, row) => sum + row.payment, 0);
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const payoffDate = addMonths(input.startDate, actualTerm);
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return {
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schedule,
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payment,
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termMonths: actualTerm,
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totalInterest: round2(totalPaid - P),
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totalAmount: round2(totalPaid),
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payoffDate,
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paidMonths: 0,
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currentBalance: schedule[schedule.length - 1]?.balance ?? 0,
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};
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}
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for (let month = 1; month <= paidMonths; month++) {
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const interest = round2(i > 0 ? balance * i : 0);
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let principalPart = round2(payment - interest);
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if (month === termMonths || principalPart > balance) {
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let actualPayment = payment;
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if (principalPart > balance) {
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principalPart = round2(balance);
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payment = round2(interest + principalPart);
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actualPayment = round2(interest + principalPart);
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}
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balance = round2(balance - principalPart);
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const date = formatDate(addMonths(input.startDate, month - 1));
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schedule.push({
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month,
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date,
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payment,
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payment: actualPayment,
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interest,
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principal: principalPart,
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balance: Math.max(0, balance),
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@@ -105,6 +237,27 @@ export function buildSchedule(input: {
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if (balance <= 0) break;
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}
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const lastHistoricalRow = schedule[schedule.length - 1];
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const projectedBalance = currentBalance;
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if (lastHistoricalRow) {
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const adjustment = round2(projectedBalance - lastHistoricalRow.balance);
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if (Math.abs(adjustment) > 0) {
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lastHistoricalRow.balance = projectedBalance;
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lastHistoricalRow.principal = round2(lastHistoricalRow.principal + adjustment);
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}
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}
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const future = buildFutureSchedule(
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projectedBalance,
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input.annualRate,
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payment,
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input.startDate,
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paidMonths + 1,
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);
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schedule.push(...future);
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const actualTerm = schedule.length;
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const totalPaid = schedule.reduce((sum, row) => sum + row.payment, 0);
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const payoffDate = addMonths(input.startDate, actualTerm);
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@@ -114,7 +267,10 @@ export function buildSchedule(input: {
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payment,
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termMonths: actualTerm,
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totalInterest: round2(totalPaid - P),
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totalAmount: round2(totalPaid),
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payoffDate,
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paidMonths,
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currentBalance: projectedBalance,
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};
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}
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