realestate-jp/esm/lib/loan/equal_principal.js

不動産関連のユーティリティ関数を提供するライブラリ

import { MONTHS_PER_YEAR } from "../constants.js";
import { BONUS_MONTHS, DEFAULT_LOAN_RATE, DEFAULT_LOAN_YEARS, } from "./constants.js";
import { calculateMonthlyInterest, isBonusMonth } from "./utils.js";
/**
 * 元金均等方式による住宅ローン返済を計算する（ボーナス加算対応）
 *
 * @param principal 借入金額（円）
 * @param rate 年利（％、デフォルト: 1.5）
 * @param years 返済年数（デフォルト: 35）
 * @param downPayment 頭金（円、デフォルト: 0）
 * @param annualBonus 年間ボーナス加算額（円、夏冬合計、デフォルト: 0）
 * @returns ローン返済結果
 *
 * @example 使用例
 * ```ts
 * const result = calcEqualPrincipal(30000000, 1.5, 35, 5000000, 200000);
 * console.log(result);
 * ```
 */
export function calcEqualPrincipal(principal, rate = DEFAULT_LOAN_RATE, years = DEFAULT_LOAN_YEARS, downPayment = 0, annualBonus = 0) {
    const loanAmount = principal - downPayment;
    const months = years * MONTHS_PER_YEAR;
    const monthlyPrincipal = loanAmount / months;
    const bonusPerMonth = annualBonus / BONUS_MONTHS.length;
    let total = 0;
    let normalSum = 0;
    let bonusSum = 0;
    let normalCount = 0;
    let bonusCount = 0;
    for (let i = 0; i < months; i++) {
        const isBonus = isBonusMonth(i);
        const monthlyInterest = calculateMonthlyInterest(loanAmount, monthlyPrincipal, i, rate);
        const basePayment = monthlyPrincipal + monthlyInterest;
        const payment = isBonus ? basePayment + bonusPerMonth : basePayment;
        total += payment;
        if (isBonus) {
            bonusSum += payment;
            bonusCount++;
        }
        else {
            normalSum += payment;
            normalCount++;
        }
    }
    const avgMonthly = normalSum / normalCount;
    const avgBonus = bonusSum / bonusCount;
    const annual = (avgMonthly * normalCount + avgBonus * bonusCount) / years;
    return {
        monthly: Math.round(avgMonthly),
        bonusMonth: Math.round(avgBonus),
        annual: Math.round(annual),
        total: Math.round(total),
    };
}