What is code coverage?

JavaScript

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.

Interview Tip

Make the point that coverage is a useful metric but not the goal. The goal is confidence that your code works. Mention branch coverage specifically — it catches more bugs than line coverage alone. If you can say "I aim for 80% coverage but focus on testing critical paths," that shows a practical mindset.

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.

Table of Contents