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Why Test Engineers Should Understand Unit Test Coverage

3 min readApr 6, 2025

There’s an ongoing debate in QA forums:
Should testers write unit tests?

…Most people say NO — and I mostly agree.

Unit tests are typically the responsibility of developers. They’re close to the code, understand the structure, and can write precise, fast, isolated tests. But here’s what I do think test automation engineers should care about:

👉 Knowing what unit tests cover.
👉 Knowing what they don’t.

So, What Exactly is a Unit Test?

Unit tests are designed to verify individual pieces of logic — a function, a method, a class — in complete isolation. They don’t connect to a real database, file system, or network. They don’t rely on the system being up and running.

✅ If a test requires the SUT (System Under Test) to be running, it’s not a unit test. That’s likely an integration test.

This distinction is important because it highlights one of the core limitations of unit tests: they don’t simulate how systems work together or how users interact with them. That’s why companies often default to UI automation — it’s the closest to real user behaviour.

But relying only on UI tests is time-consuming, expensive, and fragile.

Example: Where Unit Test Awareness Saves Time

Let’s say you have a business rule:
“If a user is under 18, they can’t register for an account.”

That logic, a simple age check, is best tested with unit tests. You can cover edge cases quickly:

  • Age exactly 18
  • Age 17
  • Invalid date formats

Unit tests cover the business logic.
So, as a test automation engineer, you could write 3–5 UI tests to verify this logic.

Or you could:

  • Confirm the logic is unit-tested (ask devs or check the code)
  • Write one UI test to simulate a happy path (e.g., user gets re-engaged)

This dramatically reduces test time, flakiness, and maintenance.

You will be right to argue that this alone will not verify that all the components will work together as expected. They don’t ensure that the correct actions will actually trigger when the business logic is applied. For that, you could add integration tests, which is a separate point for discussion.

But what the unit tests do ensure is that the core business rules are functioning properly in isolation. And understanding their coverage allows you to focus UI tests on user interactions and end-to-end experiences, rather than duplicating logic already covered by unit tests.

Conclusion: Test Strategically, Not Excessively

As testers it can be hard not to over-test. The urge to cover everything is real. But more tests don’t always mean better coverage. They can mean more flakiness, longer runtimes and wasted effort.

When test automation engineers know what unit tests already verify:

  • They avoid duplicating logic unnecessarily
  • They reduce test execution time and maintenance overhead
  • They focus UI tests on what really matters: the user experience

You don’t need to write unit tests to benefit from them. But you do need to be aware of what they cover.

Mariam Sargsyan
Mariam Sargsyan

Written by Mariam Sargsyan

Sharing insights on the testing world!

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