Testing & mocks

Test your application boundary

Inject the operation your product needs instead of constructing a live SDK client inside business logic. A deterministic fake lets you verify the inputs and outcomes that matter without imitating every internal detail of a provider library.

type EmailInput = { to: string; subject: string };
type EmailSender = {
  send(input: EmailInput): Promise<{ id: string }>;
};

export async function sendWelcome(
  sender: EmailSender,
  recipient: string,
) {
  return sender.send({
    to: recipient.trim(),
    subject: 'Welcome',
  });
}

export function createFakeEmailSender() {
  const calls: EmailInput[] = [];
  const sender: EmailSender = {
    async send(input) {
      calls.push({ ...input });
      return { id: 'email_fixture_001' };
    },
  };
  return { sender, calls };
}

Make fixtures deterministic

Use fixed identifiers and explicit error fixtures. Inject clocks, ID generators, and random-number sources when the behavior depends on them. Create a fresh fake for each test so execution order cannot change the result.

import test from 'node:test';
import assert from 'node:assert/strict';
import { createFakeEmailSender, sendWelcome } from './email';

test('welcome sends to the normalized recipient', async () => {
  const { sender, calls } = createFakeEmailSender();
  const result = await sendWelcome(
    sender,
    '  reader@example.com  ',
  );

  assert.equal(result.id, 'email_fixture_001');
  assert.deepEqual(calls, [{
    to: 'reader@example.com',
    subject: 'Welcome',
  }]);
});

The example uses the built-in Node.js test runner. Run TypeScript through your project's configured test toolchain. The fake records what the welcome workflow actually sends, so the test checks recipient normalization and the product's chosen subject.

Cover the difficult paths

Separate live contract checks

Use dedicated credentials and test data, set a spending limit, and clean up created records. Require an explicit opt-in for tests that contact a provider. Never let a missing fixture or environment variable quietly turn a local test into a live request.