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What is unit testing?

Frontend ToolsJunior~1 min read
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Unit testing is a testing method in which the smallest parts of a program (units) are checked: individual functions, methods or modules. The goal is to make sure that each part works correctly and predictably in isolation from the rest of the code.

Why it is needed

Unit tests help with:

  • Early bug detection - a bug is caught right away in a small fragment, not in a huge system.
  • Confidence in changes - you can safely refactor code knowing that the tests will catch a mistake.
  • Reliability and stability of the project - fewer unexpected breakages.
  • Documentation of behavior - the tests show how a function is supposed to work.

What it looks like in a simple example

There is a function:

js
function sum(a, b) { return a + b; }

Unit test:

js
test("sum adds 2 + 3 to equal 5", () => { expect(sum(2, 3)).toBe(5); });

If the function breaks, the test shows it right away.

What exactly gets tested in unit tests

  • the correct result
  • behavior with invalid data
  • behavior at boundaries (0, empty strings, large numbers, etc.)
  • absence of side effects

How unit tests differ from other tests

Test typeScope of the check
Unita small function → does it work correctly?
Integrationseveral modules → do they work correctly together?
E2E (end-to-end)the whole product → does it pass a real user scenario?

Summary

Unit testing is an automated check of individual functions and modules to make sure that every small piece of a program works the way it should.

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