Choose the test that can actually prove the claim

First, build a mental model

A test surrounds a contract with controlled inputs and observable outputs. Its authority ends at the boundaries it replaces with mocks.

1. Unit tests isolate small logic contracts. Integration tests exercise cooperating components such as a data adapter and database. E2E tests follow user-visible flows through the deployed-style application.
2. A mock is a controlled substitute, not evidence that the substituted system behaves as imagined. Use it to isolate a question and document the boundary it removes.
3. Fixtures should be small, named and isolated. Include missing, malformed, boundary and unauthorized cases that could falsify the claim.
4. Coverage reports tell you which code ran, not whether assertions detected wrong behavior. A test pyramid is a cost/feedback guide, not a quota to satisfy blindly.

import test from 'node:test';
import assert from 'node:assert/strict';

test('one toy order line totals integer cents', () => {
  const total = 125 * 2;
  assert.equal(total, 250);
});
// This proves the stated arithmetic only, not RLS or persistence.

1. Claim

   The example checks one calculation and an independent expected value.

2. Boundary

   No HTTP request, database policy or browser interaction was exercised.

3. Next proof

   If the claim is 'unauthorized orders cannot be saved', choose a test that reaches the real write authority.