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Unit testing

Overview​

Unit tests exercise small pieces of code: usually a single function or class, in isolation, using mocks or stubs for collaborators. Fast, deterministic unit suites are the foundation of refactoring confidence and short feedback loops in CI.

Key concepts​

  • AAA pattern: Arrange, Act, Assert.
  • Isolation: Control time, I/O, and randomness; avoid flakiness.
  • Test doubles: Mocks, fakes, stubs, spies (terminology varies by framework).
  • Coverage: Line/branch coverage is a signal, not a goal.
  • Naming: Tests document expected behavior for future readers.

Unit test flow​

Sample: Jest-style test​

import { sum } from './math';

describe('sum', () => {
  it('adds two numbers', () => {
    expect(sum(2, 3)).toBe(5);
  });
});

References​