How do I write effective unit tests?

Effective unit testing guide:

What is Unit Testing?

Testing individual units of code in isolation to verify they work correctly.

Best Practices

1. Test One Thing

Each test should verify one specific behavior.

2. Use AAA Pattern

Arrange (setup), Act (execute), Assert (verify).

3. Meaningful Names

Test names should describe what is being tested and expected outcome.

4. Keep Tests Fast

Fast tests encourage running them often.

5. Test Edge Cases

Test boundaries, nulls, empty values, errors.

6. Mock Dependencies

Isolate units by mocking external dependencies.

7. Aim for Coverage

Target 80 percent coverage but focus on critical paths.

8. Test First (TDD)

Consider writing tests before code for better design.

Tools

PHPUnit for PHP, Jest for JavaScript, JUnit for Java, pytest for Python.

I want to start testing my code but I am not sure how to write good unit tests. What are the best practices for creating effective and meaningful tests?