initial: rent vs buy calculator

calc.js

@@ -0,0 +1,121 @@
+/**
+ * calc.js — pure calculation engine, no DOM
+ */
+
+/**
+ * Run the full rent vs buy simulation month by month.
+ * Returns arrays (one value per year) plus summary stats.
+ *
+ * @param {Object} p - input parameters
+ * @returns {Object} result
+ */
+function run_simulation(p) {
+  const months = p.years * 12;
+
+  // --- mortgage setup ---
+  const loan = p.home_price * (1 - p.down_pct / 100);
+  const r_m = p.interest_rate / 100 / 12;
+  const n = p.loan_years * 12;
+  let monthly_payment;
+  if (r_m === 0) {
+    monthly_payment = loan / n;
+  } else {
+    monthly_payment = loan * (r_m * Math.pow(1 + r_m, n)) / (Math.pow(1 + r_m, n) - 1);
+  }
+
+  // upfront costs for buyer
+  const down_payment = p.home_price * (p.down_pct / 100);
+  const upfront_buy = down_payment + p.home_price * (p.closing_buy_pct / 100);
+
+  // --- state ---
+  let balance = loan;
+  let home_value = p.home_price;
+  let portfolio = upfront_buy; // renter invests this lump sum
+  let monthly_rent = p.monthly_rent;
+
+  let cum_buy_cost = upfront_buy; // running cash spent on buying
+  let cum_rent_cost = 0;
+
+  // per-year arrays (index 0 = end of year 1)
+  const buyer_nw_arr = [];
+  const renter_nw_arr = [];
+  const cum_buy_arr = [];
+  const cum_rent_arr = [];
+  const labels = [];
+
+  let breakeven_month = null;
+
+  const invest_r_m = p.invest_return / 100 / 12;
+  const app_r_m = p.appreciation / 100 / 12;
+  const rent_inc_m = p.rent_increase / 100 / 12;
+
+  for (let m = 1; m <= months; m++) {
+    // --- buy side ---
+    const interest_m = balance * r_m;
+    const principal_m = monthly_payment - interest_m;
+    balance = Math.max(0, balance - principal_m);
+    home_value *= (1 + app_r_m);
+
+    const prop_tax_m = home_value * (p.prop_tax_rate / 100) / 12;
+    const maint_m = home_value * (p.maint_rate / 100) / 12;
+    const insurance_m = p.insurance / 12;
+    // German context: no mortgage interest tax deduction for private buyers
+    const total_buy_m = monthly_payment + prop_tax_m + maint_m + insurance_m;
+
+    cum_buy_cost += total_buy_m;
+
+    const selling_costs = home_value * (p.closing_sell_pct / 100);
+    const buyer_nw = home_value - selling_costs - balance;
+
+    // --- rent side ---
+    const renters_ins_m = p.renters_ins / 12;
+    const total_rent_m = monthly_rent + renters_ins_m;
+    cum_rent_cost += total_rent_m;
+
+    // renter invests the difference if buying is more expensive
+    const monthly_delta = Math.max(0, total_buy_m - total_rent_m);
+    portfolio = portfolio * (1 + invest_r_m) + monthly_delta;
+
+    // apply capital gains tax on portfolio gains when we "cash out" at end
+    // (we track gross portfolio, deduct tax in summary)
+    const renter_nw = portfolio;
+
+    // breakeven: when buyer net worth overtakes renter
+    if (breakeven_month === null && buyer_nw >= renter_nw) {
+      breakeven_month = m;
+    }
+
+    // rent increases monthly
+    monthly_rent *= (1 + rent_inc_m);
+
+    // record yearly snapshots
+    if (m % 12 === 0) {
+      const yr = m / 12;
+      labels.push(`Jahr ${yr}`);
+      buyer_nw_arr.push(Math.round(buyer_nw));
+      // apply Abgeltungssteuer on portfolio gains at cashout
+      const portfolio_gains = portfolio - upfront_buy;
+      const tax_on_gains = Math.max(0, portfolio_gains) * (p.tax_rate / 100);
+      renter_nw_arr.push(Math.round(portfolio - tax_on_gains));
+      cum_buy_arr.push(Math.round(cum_buy_cost));
+      cum_rent_arr.push(Math.round(cum_rent_cost));
+    }
+  }
+
+  // final net worth at end of horizon
+  const final_buyer_nw = buyer_nw_arr[buyer_nw_arr.length - 1];
+  const final_renter_nw = renter_nw_arr[renter_nw_arr.length - 1];
+
+  return {
+    labels,
+    buyer_nw_arr,
+    renter_nw_arr,
+    cum_buy_arr,
+    cum_rent_arr,
+    monthly_payment: Math.round(monthly_payment),
+    final_buyer_nw,
+    final_renter_nw,
+    breakeven_month,
+    winner: final_buyer_nw >= final_renter_nw ? 'buy' : 'rent',
+  };
+}