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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