Code coverage basics in Node.js - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the code coverage tool 'nyc' in a Node.js project.

Node.js

const nyc = require('[1]');

Drag options to blanks, or click blank then click option'

Amocha

Bnyc

Cjest

Dchai

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'mocha' or 'jest' instead of 'nyc' for coverage import.

Forgetting to install 'nyc' before importing.

The 'nyc' package is the standard tool for code coverage in Node.js. Importing it correctly allows you to use its features.

2fill in blank

medium

Complete the command to run tests with coverage using 'nyc' and 'mocha'.

Node.js

npm [1]

Drag options to blanks, or click blank then click option'

Atest

Brun

Cexec

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'run' or 'start' instead of 'test' in the command.

Forgetting to configure the package.json 'test' script with 'nyc mocha'.

The 'test' command runs the test scripts. Using 'npm test' runs tests with coverage.

3fill in blank

hard

Fix the error in the coverage configuration by completing the missing key in the JSON.

Node.js

{
  "[1]": "./src",
  "reporter": ["text", "html"]
}

Drag options to blanks, or click blank then click option'

Ainclude

Bexclude

Csource

DcoverageDir

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'exclude' instead of 'include' causes wrong coverage files.

Using 'source' which is not a valid key.

The 'include' key tells 'nyc' which files to include for coverage. It should point to the source folder.

4fill in blank

hard

Fill both blanks to create a script in package.json that runs tests with coverage and outputs an HTML report.

Node.js

"scripts": {
  "coverage": "nyc --reporter=[1] [2]"
}

Drag options to blanks, or click blank then click option'

Ahtml

Bmocha

Ctext

Djest

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'text' instead of 'html' for the reporter when HTML output is needed.

Using 'jest' instead of 'mocha' if the project uses mocha.

The 'html' reporter generates an HTML report. 'mocha' runs the tests. Together they produce coverage with an HTML report.

5fill in blank

hard

Fill all three blanks to create a coverage ignore configuration that excludes test files and node_modules.

Node.js

{
  "exclude": ["[1]", "[2]", "[3]"]
}

Drag options to blanks, or click blank then click option'

Atest

Bnode_modules

C*.spec.js

Dsrc

Attempts:

3 left

💡 Hint

Common Mistakes

Including 'src' in exclude which removes source files from coverage.

Not excluding test files causing coverage to include tests.

To ignore coverage on test files and dependencies, exclude 'node_modules', 'test' folder, and '*.spec.js' files.

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