test(format): Add unit tests for XLM amount formatter (#39)

lib/__tests__/formatXLM.test.ts

@@ -0,0 +1,39 @@
+import { describe, it, expect } from "vitest";
+import { formatXLM } from "../format";
+
+describe("formatXLM amount formatter", () => {
+  it("formats 10000 XLM with thousand separators and 2 decimal places", () => {
+    expect(formatXLM(10000)).toBe("10,000.00 XLM");
+  });
+
+  it("formats 0 XLM as '0.00 XLM'", () => {
+    expect(formatXLM(0)).toBe("0.00 XLM");
+  });
+
+  it("formats fractional amounts (0.5 XLM) to 2 decimal places ('0.50 XLM')", () => {
+    expect(formatXLM(0.5)).toBe("0.50 XLM");
+  });
+
+  it("formats large numbers (1000000 XLM) with thousand separators ('1,000,000.00 XLM')", () => {
+    expect(formatXLM(1000000)).toBe("1,000,000.00 XLM");
+  });
+
+  it("formats negative values with a leading minus sign ('-500.00 XLM')", () => {
+    expect(formatXLM(-500)).toBe("-500.00 XLM");
+  });
+
+  it("handles string inputs cleanly", () => {
+    expect(formatXLM("10000")).toBe("10,000.00 XLM");
+    expect(formatXLM("0.5")).toBe("0.50 XLM");
+    expect(formatXLM("-500")).toBe("-500.00 XLM");
+  });
+
+  it("avoids floating-point representation rounding errors", () => {
+    expect(formatXLM(0.1 + 0.2)).toBe("0.30 XLM");
+    expect(formatXLM(1234.5678)).toBe("1,234.57 XLM");
+  });
+
+  it("handles invalid or NaN inputs gracefully with fallback", () => {
+    expect(formatXLM("invalid")).toBe("0.00 XLM");
+  });
+});

lib/format.ts

@@ -0,0 +1,18 @@
+/**
+ * Formats a numeric or string XLM amount with thousand separators,
+ * 2 decimal places, and trailing " XLM" suffix.
+ * Handles negative values and prevents floating-point representation errors.
+ */
+export function formatXLM(amount: number | string | bigint): string {
+  const num = typeof amount === "string" ? Number(amount) : Number(amount);
+  if (isNaN(num)) {
+    return "0.00 XLM";
+  }
+
+  const formatted = num.toLocaleString("en-US", {
+    minimumFractionDigits: 2,
+    maximumFractionDigits: 2,
+  });
+
+  return `${formatted} XLM`;
+}

lib/portfolio.ts

@@ -1,3 +1,5 @@
+import { formatXLM } from "./format";
+
 export interface InvestmentPosition {
     invoice_id: string;
     invoice_title: string;
@@ -21,9 +23,6 @@ export function calculateActiveTotal(positions: InvestmentPosition[]): Portfolio
 
     return {
         activeTotal,
-        formattedTotal: `${activeTotal.toLocaleString(undefined, {
-            minimumFractionDigits: 2,
-            maximumFractionDigits: 2,
-        })} XLM`,
+        formattedTotal: formatXLM(activeTotal),
     };
 }