fix: prevent Infinity in effectiveAnnualReturn when initialBalance is 0

When initialBalance is 0, the effectiveAnnualReturn calculation was
dividing by zero, resulting in Infinity. Now handles this edge case by:
- Using totalContributions as the base when initialBalance is 0
- Returning 0 when both initialBalance and totalContributions are 0
- Maintaining original calculation for non-zero initial balances

This ensures accurate return metrics for accounts starting from zero.

Author: introvertedspud

package.json

@@ -1,6 +1,6 @@
 {
   "name": "retirement-calculator",
-  "version": "1.1.0",
+  "version": "1.1.1",
   "description": "A versatile retirement financial planning tool for calculating savings, inflation impact, and withdrawal strategies.",
   "main": "dist/index.js",
   "types": "dist/index.d.ts",

src/RetirementCalculator.ts

@@ -545,8 +545,16 @@ export default class RetirementCalculator {
     }
 
     // Calculate summary statistics
-    const effectiveAnnualReturn =
-      Math.pow(balance / initialBalance, 1 / totalYears) - 1;
+    let effectiveAnnualReturn: number;
+    if (initialBalance === 0) {
+      // When starting from 0, calculate return based on contributions instead
+      effectiveAnnualReturn =
+        totalContributions > 0
+          ? Math.pow(balance / totalContributions, 1 / totalYears) - 1
+          : 0;
+    } else {
+      effectiveAnnualReturn = Math.pow(balance / initialBalance, 1 / totalYears) - 1;
+    }
     const averageMonthlyReturn =
       monthlyTimeline.reduce(
         (sum, entry) => sum + entry.currentMonthlyReturn,