Testing in the Pipeline

• Push each check to the lowest level that can still catch the bug
• Logic bug: unit test, not a browser run
• Wiring or schema bug: integration test
• Only critical user flows earn an E2E test

• Why not test everything E2E? Slow, costly, flaky, hard to debug
• When pick the trophy? When units are thin glue code

• Unit: one function or class, dependencies swapped for mocks, stubs or fakes
• Runs in milliseconds, no network or disk
• Integration: several real components together, e.g. service + real database
• Catches wiring, schema and config bugs
• E2E: whole system through the UI or public API, like a user
• Playwright or Cypress on a login-to-checkout flow

• Everything mocked: tests pass, production breaks
• Asserting internal calls breaks on every refactor

assert order.total == 42  # output: good
db.save.assert_called_once()  # internal: avoid

check what the code returns or does

eslint . --fix
ruff check .
mypy src/
tsc --noEmit

--fix rewrites files; --noEmit only checks types

• Local hooks can be skipped, so CI is the enforcer
• Run the same checks in both places

• Blanket-disabling a rule for the whole project
• Ignore comments with no written reason

• Coverage = share of code executed by tests
• Line: lines run. Function: functions called
• Branch: every if/else path, so stricter than line
• One test can hit a line yet skip the other branch

def fee(n):
    if n > 100:
        n -= 10
    return n
assert fee(200) == 190

100% line, but the False branch never ran

• Reports come out as lcov or HTML
• Use the report to find untested paths

• 100% does not prove correctness: executed is not asserted
• Assertion-free tests only chase the number
• Check test strength with mutation testing: Stryker, mutmut

• Flaky test = passes and fails on the same code
• Eroded trust: people stop believing red builds

• Quality gate = automated pass/fail in CI that blocks merge
• Tests green, lint clean, type check
• Coverage on new code at or above threshold
• No critical vulnerabilities

• Slow or noisy gates get bypassed
• Run fast checks first; require them on protected branches
• Gate new code, not the whole legacy base

• Why not auto-retry flaky tests? It hides the cause and erodes trust.
• What does a gate block? Merge, unless tests, lint, types, coverage pass.

• Run static checks in pre-commit and again in CI

eslint . --fix
ruff check .
mypy src/
tsc --noEmit
pytest --cov=src --cov-branch
jest --coverage

lint, types, then coverage

• Prefer branch coverage over line coverage
• Gate new code, not the whole legacy base
• Check test strength with mutation testing

• No sleeps: wait on conditions
• Isolate state per test
• Seed randomness, fix clocks
• Quarantine flaky tests, never auto-retry