fix: handle division by zero in calculator divide function

Throw an Error with message 'Division by zero' when the divisor is
zero, and add tests covering both normal division and the error case.

Author: QuantCode Agent

src/calculator.ts

@@ -15,7 +15,9 @@ export function multiply(a: number, b: number): number {
   return a * b
 }
 
-// BUG: Division by zero is not handled
 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 +13,12 @@ describe("calculator", () => {
   test("multiplies two numbers", () => {
     expect(multiply(4, 3)).toBe(12)
   })
+
+  test("divides two numbers", () => {
+    expect(divide(10, 2)).toBe(5)
+  })
+
+  test("throws on division by zero", () => {
+    expect(() => divide(10, 0)).toThrow("Division by zero")
+  })
 })