Testing - lesson 4 | HackIndia Skills

Testing - lesson 4 | HackIndia Skills | HackIndia

Example

Small Node example

Here the code sends an OTP only after saving it. The test fakes the SMS service because sending a real SMS costs money and can fail for reasons outside your code.

The fake records what would have been sent. The test still checks real app behaviour.

// otp.js
async function sendLoginOtp(phone, store, smsClient) {
  const otp = "123456";
  await store.saveOtp(phone, otp);
  await smsClient.send(phone, `Your OTP is ${otp}`);
  return { ok: true };
}

module.exports = { sendLoginOtp };

// otp.test.js
const { sendLoginOtp } = require("./otp");

test("saves OTP and asks SMS client to send it", async () => {
  const saved = [];
  const sent = [];

  const fakeStore = {
    saveOtp: async (phone, otp) => saved.push({ phone, otp })
  };

  const fakeSmsClient = {
    send: async (phone, message) => sent.push({ phone, message })
  };

  const result = await sendLoginOtp("9876543210", fakeStore, fakeSmsClient);

  expect(result).toEqual({ ok: true });
  expect(saved).toEqual([{ phone: "9876543210", otp: "123456" }]);
  expect(sent[0].phone).toBe("9876543210");
  expect(sent[0].message).toContain("123456");
});

Use test doubles safely

One idea

What doubles replace

Common mistake

Use safely

Small Node example

AI check

Do not fake contracts

Do this now

Quick check