What is test coverage and how can it be measured?

Test coverage is a metric that shows which part of the code is covered by automated tests. It helps assess the completeness of testing and identify untested areas.

Main types of coverage:

• Line coverage — how many lines of code are executed during tests.
• Branch coverage — how many logical branches (if/else) are tested.
• Function coverage — how many functions are called.
• Condition coverage — testing all conditions within expressions.

Coverage can be measured using tools such as:

• Istanbul/nyc for JavaScript/TypeScript
• JaCoCo for Java
• Coverage.py for Python

These analyze test execution and generate reports with coverage percentages and file-level details.

Example command for measuring coverage with nyc:

nyc mocha

Where mocha is the test framework. The result is a report with coverage percentages.