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.

Practice

1. What does code coverage measure in a Node.js project?

easy

A. How much of the code is executed by tests

B. The speed of the Node.js server

C. The number of users accessing the app

D. The size of the project files

Solution

1. Step 1: Understand the purpose of code coverage

   Code coverage shows which parts of your code are run when tests execute.

2. Step 2: Identify what code coverage measures

   It measures the percentage of code lines or branches tested, not speed or size.

3. Final Answer:

   How much of the code is executed by tests -> Option A

4. Quick Check:

   Code coverage = executed code percentage [OK]

Hint: Code coverage = tested code parts percentage [OK]

Common Mistakes:

• Confusing coverage with performance
• Thinking coverage measures user count
• Assuming coverage measures file size

2. Which command is commonly used to run tests with coverage in a Node.js project using nyc?

easy

A. nyc node test.js

B. nyc mocha

C. node --coverage test.js

D. npm run coverage

Solution

1. Step 1: Recall nyc usage with test runners

   nyc is a tool that wraps test commands like mocha to collect coverage.

2. Step 2: Identify correct syntax

   Running nyc mocha runs mocha tests with coverage collection.

3. Final Answer:

   nyc mocha -> Option B

4. Quick Check:

   nyc + test runner = coverage run [OK]

Hint: Use nyc before test command to collect coverage [OK]

Common Mistakes:

• Using nyc without a test runner
• Trying node --coverage (not standard)
• Assuming npm run coverage always exists

3. Given this code and test, what coverage report will nyc show for the function add?

function add(a, b) {
  if (a > 0) {
    return a + b;
  } else {
    return 0;
  }
}

// Test
add(2, 3);

medium

A. 75% coverage - partial lines executed

B. 100% coverage - all lines executed

C. 0% coverage - function not called

D. 50% coverage - else branch not executed

Solution

1. Step 1: Analyze which branches run

   The test calls add(2, 3), so the if condition (a > 0) is true, running the first return.

2. Step 2: Identify uncovered code

   The else branch is never executed, so that part is not covered.

3. Final Answer:

   50% coverage - else branch not executed -> Option D

4. Quick Check:

   Uncovered else branch = 50% coverage [OK]

Hint: Check if all branches run to get full coverage [OK]

Common Mistakes:

• Assuming all code runs if function is called once
• Ignoring branches in coverage
• Confusing line coverage with function call

4. You ran nyc mocha but the coverage report shows 0% coverage. What is the most likely cause?

medium

A. The source files are excluded in nyc config

B. nyc was not installed

C. Mocha is not compatible with nyc

D. Tests did not run any code

Solution

1. Step 1: Understand why coverage can be zero

   If source files are excluded in nyc config, coverage won't track them.

2. Step 2: Check other options

   Tests running but no coverage means exclusion is likely; nyc installed and mocha compatible.

3. Final Answer:

   The source files are excluded in nyc config -> Option A

4. Quick Check:

   Excluded files cause zero coverage [OK]

Hint: Check nyc config excludes before blaming tests [OK]

Common Mistakes:

• Assuming tests never ran
• Ignoring nyc config exclusions
• Thinking nyc and mocha are incompatible

5. You want to improve coverage for this function:

function checkValue(x) {
  if (x) {
    return 'Yes';
  } else {
    return 'No';
  }
}

Your tests only call checkValue(true). What change will increase coverage?

hard

A. Add a test calling checkValue('Yes')

B. Add a test calling checkValue(1)

C. Add a test calling checkValue(false)

D. No change needed, coverage is already 100%

Solution

1. Step 1: Identify which branches are tested

   Calling checkValue(true) runs the if branch only, not the else branch.

2. Step 2: Determine how to cover else branch

   Passing false triggers the else branch, increasing coverage.

3. Final Answer:

   Add a test calling checkValue(false) -> Option C

4. Quick Check:

   Test both true and false for full branch coverage [OK]

Hint: Test both true and false to cover all branches [OK]

Common Mistakes:

• Testing other truthy values
• Assuming one test covers all branches
• Ignoring falsy values in coverage