All 10 Debugging, Testing & Tooling questions
More in Debugging, Testing & Tooling
How do you pause code execution at a specific line using the browser DevTools? What is console.table() useful for? What is a unit test and what makes a good one? How does Jest's mocking help isolate the code under test? How does source mapping help when debugging minified or transpiled code? How do you write an effective test for asynchronous code? What is snapshot testing and what's a common pitfall with it? How does the Performance panel in DevTools help diagnose a janky UI? What's the difference between end-to-end (E2E) tests and unit tests, and when do you use each?
Advertisement
Back to results for “Testing & Debugging”
JavaScript › Debugging, Testing & Tooling

What is code coverage and why isn't 100% coverage the same as bug-free code?

Intermediate

Answer

Code coverage measures which lines/branches of code ran during tests, expressed as a percentage. High coverage only proves code was *executed*, not that its behavior was *correctly asserted* — a test can run every line and still assert nothing meaningful.
// A test that runs the code but checks nothing:
test('runs without checking', () => {
  calculateTotal(items); // 100% line coverage, zero verification
});
Real-world example A team hitting 100% coverage but still shipping bugs because tests lacked real assertions.

Common follow-ups: What's the difference between line coverage and branch coverage?

Error Handling
All 10 Debugging, Testing & Tooling questions