Code coverage basics in Node.js - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Understanding coverage output summary

Given the following test coverage summary output from a Node.js project, what is the percentage of statements covered?

Node.js

Statements   : 80.0% ( 40/50 )
Branches     : 75.0% ( 15/20 )
Functions    : 90.0% ( 9/10 )
Lines       : 85.0% ( 34/40 )

A 80%

B 75%

C 90%

D 85%

💡 Hint

Look for the line labeled 'Statements' and check the percentage.

The 'Statements' line shows 80.0% coverage, meaning 40 out of 50 statements were executed during tests.

❓ component_behavior

intermediate

Effect of missing tests on coverage

If a function in your Node.js code is never called by any test, what will the coverage report show for that function?

A The coverage report will show an error.

B The function will show 100% coverage by default.

C The function will show 0% coverage.

D The function will be excluded from the coverage report.

💡 Hint

Think about what happens if code is never executed during tests.

If a function is never called during tests, coverage tools mark it as uncovered, showing 0% coverage.

📝 Syntax

advanced

Correct way to instrument code for coverage

Which of the following commands correctly instruments a Node.js project using nyc for coverage before running tests?

A mocha --coverage

B nyc --reporter=lcov mocha

C node --coverage test.js

D npm run coverage

💡 Hint

nyc is a popular coverage tool that wraps test commands.

The command 'nyc --reporter=lcov mocha' runs mocha tests with nyc coverage instrumentation and outputs lcov reports.

🔧 Debug

advanced

Diagnosing missing coverage data

You ran your tests with coverage, but the report shows 0% coverage for all files. What is the most likely cause?

A The source files are excluded in the coverage configuration.

B The coverage tool was not installed.

C The coverage report was generated before running tests.

D The tests did not execute any code because they failed to run.

💡 Hint

Verify if tests ran successfully and executed the source code.

If the tests did not execute any code (e.g., because they failed to run), the coverage report will show 0% for all instrumented files.

🧠 Conceptual

expert

Interpreting branch coverage results

A coverage report shows 75% branch coverage. What does this mean about your tests?

A 75% of all possible paths through conditional branches were executed during tests.

B 75% of the code lines were executed during tests.

C 75% of functions were called during tests.

D 75% of statements were executed during tests.

💡 Hint

Branch coverage relates to conditional paths, not lines or functions.

Branch coverage measures how many possible paths through if/else or switch statements were tested.