Summary
Introduction
Have you ever written a test that sometimes passes and sometimes fails for no clear reason? Or maybe your test suite takes so long to run that you avoid running it altogether?
These are common struggles in software testing—and they’re exactly what the FIRST principles are designed to solve.
FIRST is an acronym that captures five key qualities of good tests. When applied, they make your tests more reliable, faster, and easier to maintain—which means fewer bugs slipping through and more confidence when shipping code.
What Are the FIRST Principles?
The acronym FIRST stands for:
- Fast
- Independent
- Repeatable
- Self-Validating
- Timely
Let’s break each one down with real-world context.
1. Fast
Tests should run quickly—ideally in seconds. Slow tests discourage developers from running them often, which defeats their purpose.
- Good practice: Keep unit tests lightweight, avoid hitting external databases or APIs.
- Bad practice: Waiting 5 minutes every time you run the test suite.
Think of tests like a seatbelt—if it’s too much of a hassle to put on, people won’t use it.
2. Independent
Each test should be able to run on its own without depending on others. If test B relies on test A to run first, you’ve got a fragile setup.
- Good practice: Every test sets up its own data and environment.
- Bad practice: A test that only works if another test created a database record beforehand.
Independent tests are easier to maintain, run in parallel, and give clearer feedback when something fails.
3. Repeatable
Tests must give the same result every time, no matter where or when you run them. A flaky test erodes trust—it’s worse than having no test at all.
- Good practice: Mock external services like payment gateways.
- Bad practice: Relying on today’s date or random values without controlling them.
A repeatable test means failures actually tell you something is wrong—not just that the moon is in a weird phase.
4. Self-Validating
A test should either pass or fail automatically. Developers shouldn’t need to dig through log files or manually inspect results to know if it worked.
- Good practice: Use clear assertions (
assertEqual,expect, etc.). - Bad practice: Printing output and asking a human to “check if it looks okay.”
Self-validating tests save time and make CI/CD pipelines possible.
5. Timely
Tests should be written at the right time, ideally before or alongside the code they validate. Writing tests too late often leads to gaps or tests that don’t really cover the important cases.
- Good practice: Use Test-Driven Development (TDD) or at least write tests soon after the code.
- Bad practice: Trying to bolt on tests weeks later, when the code is already complex and hard to test.
Timely tests guide design, making your code more modular and testable.
Why FIRST Matters
By following the FIRST principles, your tests become:
- Reliable → They catch real bugs instead of producing noise.
- Efficient → They run fast enough to be used during active development.
- Maintainable → They remain valuable over time without becoming a burden.
This leads to better developer productivity and safer deployments.
Real-World Example
Imagine you’re building a login system:
- A Fast test checks password hashing in memory (no DB calls).
- An Independent test ensures a failed login attempt doesn’t rely on previous success tests.
- A Repeatable test uses a fixed username/password combo instead of random data.
- A Self-Validating test asserts “login succeeds with correct credentials.”
- A Timely test is written before rolling out the login feature to production.
The result? You can ship confidently without late-night bug hunts.
Conclusion
FIRST isn’t just a checklist—it’s a mindset. When your tests are fast, independent, repeatable, self-validating, and timely, you spend less time fighting flaky tests and more time building features that matter.
Next time you’re writing tests, ask yourself: Does this follow FIRST? If not, it’s worth revisiting.