property.mortgage-affordability

Imports name this capability’s declared dependencies, which fune builds next to it in your project; each one links to its page.

import { affordability } from "./lending_affordability.ts";  ← from lending.affordability ^1.0.0 · built alongside by fune
import { roundDiv } from "./math_round_div.ts";  ← from math.round-div ^1.0.0 · built alongside by fune
import { type Money, money } from "./money_amount.ts";  ← from money.amount ^1.0.0 · built alongside by fune
import { type MortgageAffordability } from "./property_mortgage_affordability_types.ts";

const MAX_INCOME = 10_000_000_000;
const MAX_MULTIPLE = 200_000;

/**
 * Loan-to-income against the lender's multiple, and the stressed repayment
 * against income via lending.affordability. Every rounding goes against the
 * borrower, and the multiple comparison itself is exact.
 */
export function mortgageAffordability(
  annualIncome: Money,
  monthlyCommitments: Money,
  loan: Money,
  annualRateBasisPoints: number,
  stressRateBasisPoints: number,
  termMonths: number,
  maxIncomeMultipleBasisPoints: number,
  maxDebtToIncomeBasisPoints: number,
): MortgageAffordability {
  if (loan.currency !== annualIncome.currency) {
    throw new RangeError(`currency mismatch: ${annualIncome.currency} and ${loan.currency}`);
  }
  if (!Number.isInteger(annualIncome.minor) || annualIncome.minor < 12 || annualIncome.minor > MAX_INCOME) {
    throw new RangeError(`annualIncome must be between 12 and ${MAX_INCOME} minor units, received ${annualIncome.minor}`);
  }
  if (!Number.isInteger(loan.minor) || loan.minor <= 0 || loan.minor > MAX_INCOME * 20) {
    throw new RangeError(`loan must be between 1 and ${MAX_INCOME * 20} minor units, received ${loan.minor}`);
  }
  const m = maxIncomeMultipleBasisPoints;
  if (!Number.isInteger(m) || m < 1 || m > MAX_MULTIPLE) {
    throw new RangeError(`maxIncomeMultipleBasisPoints must be between 1 and ${MAX_MULTIPLE}, received ${m}`);
  }
  const currency = annualIncome.currency;
  const monthlyIncome = money(Math.floor(annualIncome.minor / 12), currency);
  const repayment = affordability(
    monthlyIncome,
    monthlyCommitments,
    loan,
    annualRateBasisPoints,
    stressRateBasisPoints,
    termMonths,
    maxDebtToIncomeBasisPoints,
  );
  const withinMultiple = loan.minor * 10000 <= annualIncome.minor * m;
  return {
    incomeMultipleBasisPoints: roundDiv(loan.minor * 10000, annualIncome.minor, "up"),
    maxLoanByMultiple: money(roundDiv(annualIncome.minor * m, 10000, "down"), currency),
    withinMultiple,
    monthlyIncome,
    repayment,
    affordable: withinMultiple && repayment.affordable,
  };
}