Regression Testing — Explained with Examples
Regression testing re-runs existing test suites after code changes to ensure that new features or fixes don’t break previously working functionality.
Regression testing is the safety net of software development. Every time you add a feature, fix a bug, or refactor code, you risk introducing new bugs (regressions). Automated regression tests catch these before they reach production.
The Regression Problem
// Original function
function calculateShipping(items) {
const weight = items.reduce((sum, item) => sum + item.weight, 0);
return weight * 0.5; // $0.50 per pound
}
// After adding "free shipping over $100" feature:
function calculateShipping(items, orderTotal) {
if (orderTotal > 100) return 0; // New feature
const weight = items.reduce((sum, item) => sum + item.weight, 0);
// BUG: changed to 0.6 accidentally during refactor!
return weight * 0.6; // Regression!
}
// Regression test catches this:
test('shipping is $0.50 per pound for orders under $100', () => {
const items = [{ weight: 10 }];
expect(calculateShipping(items, 50)).toBe(5); // Should be 10 * 0.50
// Test fails — shipping is now $6.00 instead of $5.00
});Building a Regression Suite
A good regression suite prioritizes:
- Critical business paths (checkout, login, payments)
- Frequently broken features
- Previously fixed bugs
- High-risk code areas
Real-World Analogy
Regression testing is like checking your whole house after renovating a bathroom. You might have installed a new sink perfectly (new feature), but while working on the pipes, you accidentally loosened a connection in the kitchen (regression). Running through the checklist — “does the kitchen faucet still work? does the toilet flush?” — catches damage from the renovation.
Automation Strategies
# Run regression suite in CI pipeline
npm run test:regression
# Example package.json
{
"scripts": {
"test:unit": "jest --testPathPattern=unit",
"test:integration": "jest --testPathPattern=integration",
"test:regression": "jest --testPathPattern=regression --bail",
"test:all": "npm run test:unit && npm run test:integration && npm run test:regression"
}
}Related Terms
Smoke Testing, Unit Testing, Integration Testing, E2E Testing, CI/CD
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