Software Engineering basics - Regression testing

Regression testing is a type of software testing that ensures that recent code changes (like bug fixes, updates, or new features) haven’t accidentally broken or altered existing functionality.

Here’s a clear breakdown:

1. Purpose

The goal of regression testing is to verify that previously developed and tested software still works as expected after changes such as:

• New features added

• Code refactoring or optimization

• Bug fixes

• Integration with new modules or systems

2. When It’s Done

Regression testing is usually done:

• After every code change, especially in agile or continuous integration environments.

• Before a release, to make sure updates didn’t introduce new defects.

3. How It’s Done

There are two main ways:

• Manual regression testing: Testers re-execute a selected set of test cases manually.

• Automated regression testing: Automation tools (like Selenium, JUnit, Cypress, or TestNG) run test scripts automatically — this is faster and more reliable for large or frequently updated projects.

4. Test Selection

You don’t always rerun every test. Teams often choose:

• Smoke tests: To check critical functionalities.

• Test suites for affected areas: Based on the parts of code that changed.

• Full regression suites: For major releases or system-wide changes.

5. Example

Suppose you add a new “discount code” feature to an e-commerce app.
Regression testing would check:

• The checkout process still works.

• The total price calculation remains correct.

• Payment and order confirmation screens still function properly.

6. Benefits

• Detects unintended side effects early

• Ensures software stability and quality

• Builds confidence before deployment

• Supports continuous delivery