What is Unit Testing? - Explanation & Meaning

Definition

Unit testing is the testing of the smallest units of code, such as individual functions, methods, or classes, in isolation from the rest of the system. The goal is to verify that each unit works correctly according to its specification.

Technical explanation

Unit tests are the foundation of the testing pyramid and are executed most frequently. They should be fast, isolated, repeatable, and self-validating (FIRST principles: Fast, Isolated, Repeatable, Self-validating, Timely). Popular test frameworks include Jest and Vitest for JavaScript/TypeScript, pytest for Python, and JUnit for Java. Mocking replaces external dependencies (databases, APIs, file systems) with controlled simulations so tests only assess the unit under test. Spies monitor whether functions are called correctly. Test fixtures provide predefined test data. Test coverage measures what percentage of code is reached by tests: line coverage, branch coverage, and function coverage. Coverage of 80% is often considered good, but qualitative tests are more important than quantitative coverage. Parameterized tests run the same test with different input values. Snapshot testing compares output with previously captured references. Property-based testing generates random inputs to discover edge cases.

How MG Software applies this

MG Software writes unit tests for all business-critical code. We use Vitest for our TypeScript/React projects and pytest for Python services. Our CI/CD pipeline mandates minimum test coverage and blocks merges when tests fail. We combine unit tests with integration tests and end-to-end tests for complete system coverage.

Practical examples

• A unit test verifying that a price calculation function returns correct results for regular prices, discounts, VAT, and edge cases like zero values or negative amounts.
• An isolated test of an authentication service using a mock database, so the test runs fast and does not depend on a real database connection.
• A CI/CD pipeline executing 500 unit tests in under 30 seconds on every commit, catching bugs immediately before code is merged.

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