Coverage

Learn test coverage measurement.

Coverage

Coverage shows what percentage of your code is tested. High coverage means your tests exercise most of the code. Jest offers a built-in coverage tool: jest --coverage.

Example: running jest --coverage shows for each file how many lines, functions and branches are tested. The coverage report highlights untested code. High coverage is a good start, but it does not guarantee quality. What matters is testing critical paths. If coverage is low, important branches are usually untested. Adding tests and raising coverage improves code reliability. Production systems aim for 80%+ coverage.

Coverage types

Coverage has three main types: lines, functions, and branches. Line coverage shows how many lines ran, function coverage how many functions were called, branch coverage how many sides of if-else branches were tested.

Example: for an if (x > 0) branch, both x > 0 and x <= 0 cases must be tested. If only one is tested, line coverage can be 100% but branch coverage is 50%. Branch coverage is the hardest and most valuable; it guarantees critical paths are tested. The coverage report shows which lines and branches were not tested. This tells you where to add tests.

Exercise Calculate coverage

Define a calcCoverage(total, tested) function. Return the percentage with return (tested / total) * 100. Print calcCoverage(100, 80) with console.log.

Show example solution

function calcCoverage(total, tested) {
  return (tested / total) * 100;
}
console.log(calcCoverage(100, 80));

Test yourself

What does coverage measure?
```
jest --coverage
```
- Memory usage
- Disk fill rate
- What percentage of code is tested
- The app's speed
Show answer

What percentage of code is tested — Coverage measures test coverage.
Complete the coverage command
```
jest --____
```
Show answer
```
jest --coverage
```
Match each type to its meaning

lines

Lines that ran
functions

Functions called
branches

If-else sides tested
--coverage

Jest flag

Next lesson: E2E tests

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