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Welcome email tests

Testing · Difficulty 5/5 · TypeScript · 59 linesOrigin: Fictional

Task

What the AI was asked to do

Write unit tests for sendWelcomeEmails(users, mailer). The function emails only the users whose email is verified, each once, with the "welcome" template. If sending to one user fails, it puts that user in the failed list and goes on with the others. It returns the ones sent in sent and the failures in failed. In tests mailer is a mock.

Handoff note

What the AI wrote when it handed over the code:

“Four tests: sending to verified users, skipping unverified ones, a failed send and the empty list. If an unverified user is emailed, the assertion in the mock fails the test at once; going on with the others after a failed send is covered too.”

The code the AI wrote

TypeScript59 lines

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  • CriticalData loss, a security hole or lost money
  • MajorThe user gets a wrong result in a case the task asks for
  • MinorA rare edge case outside the task, or readability

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import assert from "node:assert/strict";
import type * as Mail from "./subject.ts";
3
const ADA = "[email protected]";
const BOB = "[email protected]";
const CY = "[email protected]";
const makeUsers = (): Mail.User[] => [
  { email: ADA, verified: true },
  { email: BOB, verified: false },
  { email: CY, verified: true },
];
12
function recordingMailer() {
  const sent: string[][] = [];
  const mailer: Mail.Mailer = {
    send: async (to, template) => {
      sent.push([to, template]);
    },
  };
  return { mailer, sent };
}
22
export async function testSendsWelcome({ sendWelcomeEmails }: typeof Mail) {
  const { mailer, sent } = recordingMailer();
  const verified = makeUsers().filter((user) => user.verified);
  const report = await sendWelcomeEmails(verified, mailer);
  assert.deepEqual(sent, [
    [ADA, "welcome"],
    [CY, "welcome"],
  ]);
  assert.deepEqual(report, { sent: [ADA, CY], failed: [] });
}
33
export async function testSkipsUnverified({ sendWelcomeEmails }: typeof Mail) {
  const mailer: Mail.Mailer = {
    send: async (to) => {
      assert.notEqual(to, BOB, "unverified users get no email");
    },
  };
  await sendWelcomeEmails(makeUsers(), mailer);
}
42
export async function testRecordsFailure({ sendWelcomeEmails }: typeof Mail) {
  const mailer: Mail.Mailer = {
    send: async (to) => {
      if (to === ADA) {
        throw new Error("mailbox full");
      }
    },
  };
  const report = await sendWelcomeEmails(makeUsers(), mailer);
  assert.deepEqual(report.failed, [ADA]);
}
54
export async function testNoUsers({ sendWelcomeEmails }: typeof Mail) {
  const { mailer } = recordingMailer();
  const report = await sendWelcomeEmails([], mailer);
  assert.deepEqual(report, { sent: [], failed: [] });
}

The test that proves it

The right option is the test that fails on this code, which means it exposes the bug.

Test options
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