fix: handle division by zero in divide() and add tests

Author: QuantCode Agent

src/calculator.ts

@@ -15,7 +15,11 @@ export function multiply(a: number, b: number): number {
   return a * b
 }
 
-// BUG: Division by zero is not handled
+/**
+ * Divides `a` by `b`.
+ * @throws {Error} When `b` is zero.
+ */
 export function divide(a: number, b: number): number {
+  if (b === 0) throw new Error("Division by zero")
   return a / b
 }

test/calculator.test.ts

@@ -1,7 +1,5 @@
 import { describe, test, expect } from "bun:test"
-import { add, subtract, multiply } from "../src/calculator"
-
-// Note: divide is intentionally not tested — has a known bug
+import { add, subtract, multiply, divide } from "../src/calculator"
 
 describe("calculator", () => {
   test("adds two numbers", () => {
@@ -15,4 +13,14 @@ describe("calculator", () => {
   test("multiplies two numbers", () => {
     expect(multiply(4, 3)).toBe(12)
   })
+
+  describe("divide", () => {
+    test("divides two numbers", () => {
+      expect(divide(10, 2)).toBe(5)
+    })
+
+    test("throws on division by zero", () => {
+      expect(() => divide(5, 0)).toThrow("Division by zero")
+    })
+  })
 })