What is code coverage?

The short answer

Code coverage measures how much of your code is executed when your tests run. It is expressed as a percentage — 80% coverage means 80% of your code lines were hit by at least one test. It helps you find untested code, but high coverage does not guarantee good tests.

Types of coverage

• Line coverage — what percentage of code lines were executed
• Branch coverage — what percentage of if/else branches were taken
• Function coverage — what percentage of functions were called
• Statement coverage — what percentage of statements were executed

Branch coverage is the most important because it catches untested conditional logic:

function getDiscount(total) {
  if (total > 100) {
    return total * 0.1;
  }
  return 0;
}

// A test that only tests total = 150 gives:
// Line coverage: 100% (all lines ran)
// Branch coverage: 50% (only the true branch)

You need a test with total = 50 to cover the other branch.

Running coverage

In Jest:

jest --coverage

This generates a report showing which lines and branches are covered.

What is a good coverage number?

• 80% is a common target for most projects
• 100% is usually not worth pursuing — the last 10-20% requires a lot of effort for diminishing returns
• Some code (error handling, edge cases) is hard to test and may not justify the effort

The more important question is: are the critical paths well tested? 80% coverage with good tests is better than 100% coverage with shallow tests.

Common Pitfalls

High code coverage does not mean your tests are good. You can have 100% coverage with tests that never check the output. Coverage tells you what code ran, not whether the code works correctly. A test that calls a function but does not assert anything gives you coverage but no confidence.

Why interviewers ask this

This tests your understanding of testing quality. Interviewers want to see if you know what coverage measures, what it does not measure, and what a reasonable target is. Candidates who chase 100% coverage or ignore coverage entirely both raise flags — the ideal answer shows a balanced, practical approach.