Common Karma Mistakes

Mistake: Missing singleRun in CI

By far the most common first-time Karma CI mistake is forgetting singleRun: true (or the --single-run flag), leaving Karma in watch mode inside a CI job that expects the process to exit. The job then hangs until the CI platform's own timeout eventually kills it, often with a confusing, generic timeout error rather than a clear Karma-specific message.

// The fix — one line, easy to miss the first time:
config.set({
  singleRun: true
});

// Or via CLI flag, without touching the config file at all:
karma start --single-run

This single setting is the most common reason a first-time Karma CI integration hangs indefinitely.

Why These Mistakes Keep Happening

1. Karma's defaults favor local development (watch mode), not CI
2. Coverage glob patterns are easy to write slightly too broadly
3. Container sandboxing issues are invisible until you actually run in a container
4. Async test mistakes (missing done()/await) often still "pass" by accident locally

• Karma's sensible local-development defaults are exactly wrong for CI — this mismatch is the root of several mistakes on this list.
• Coverage over-instrumentation is a glob pattern precision problem, not a conceptual misunderstanding.
• Container issues are genuinely invisible until the first real containerized run — proactive flags prevent the surprise.
• Some async mistakes are silent, not loud, which is exactly why they're dangerous.

Common Mistakes Cheat Sheet

Ranked roughly by how often each one actually occurs in real projects.

Deep Dive: The Silent Async Test Mistake

An async spec that forgets to return or await a promise can appear to pass even when the assertion inside it actually failed — because the test framework considers the spec complete before the assertion runs. This is one of the most dangerous mistakes precisely because it produces false confidence rather than an obvious error.

// Dangerous: appears to pass regardless of the actual assertion result
it('resolves with user data', function() {
  fetchUser(1).then(function(user) {
    expect(user.name).toBe('Ada'); // may run after Jasmine already considered this spec done
  });
});

// Correct: the framework waits for the returned promise
it('resolves with user data', async function() {
  const user = await fetchUser(1);
  expect(user.name).toBe('Ada');
});

The fix is one keyword (async/await or an explicit return), but the bug it prevents is genuinely dangerous.

Deep Dive: Why Container Launch Failures Feel Sudden

A Karma suite can run perfectly on every developer's laptop for months, then fail immediately the first time it runs inside a Docker container (locally or in CI) — because laptops don't impose the same sandbox/kernel restrictions containers do. This makes the failure feel sudden and confusing, even though it's a well-understood, documented issue with a known fix.

Common Mistakes

• Treating each of these mistakes as isolated incidents instead of recognizing the recurring root causes behind them.
• Fixing the symptom (adding a flag) without understanding why the underlying default didn't work in the first place.
• Not sharing these lessons with new team members, letting the same mistakes repeat across a growing team.
• Assuming a mistake 'can't happen to us' because the suite currently passes, rather than proactively preventing it.

Key Takeaways

• Missing singleRun: true in CI is the single most common first-time Karma CI mistake.
• Coverage over-instrumentation and container sandbox failures both stem from Karma's local-development-first defaults.
• Silent async mistakes are especially dangerous because they produce false confidence, not obvious errors.
• Understanding *why* each mistake happens prevents its variants, not just the exact scenario described here.