Code coverage basics in Node.js

Introduction

Code coverage shows how much of your code is tested by your tests. It helps find parts of code that need more testing.

When you want to check if your tests cover all important parts of your code.

Before releasing software to make sure critical code is tested.

When adding new features to ensure new code is tested.

To find unused or dead code that can be cleaned up.

When improving test quality and reliability.

Syntax

Node.js

npx nyc node your_test_file.js

# or with npm scripts
"scripts": {
  "test": "nyc mocha"
}

nyc is a popular tool to measure code coverage in Node.js.

You run your tests with nyc to collect coverage data automatically.

Examples

Run a single test file with coverage using nyc.

Node.js

npx nyc node test.js

Add a test script in package.json to run tests with coverage.

Node.js

"scripts": {
  "test": "nyc mocha"
}

Run tests and generate an HTML report showing coverage details visually.

Node.js

npx nyc --reporter=html mocha

Sample Program

This simple Node.js script has two functions and tests for both. Running it with nyc will show coverage for add and multiply.

Node.js

const assert = require('assert');

function add(a, b) {
  return a + b;
}

function multiply(a, b) {
  return a * b;
}

// Simple tests
assert.strictEqual(add(2, 3), 5);
assert.strictEqual(multiply(2, 3), 6);

console.log('Tests passed');

Important Notes

Code coverage does not guarantee your code is bug-free, only that tests run those parts.

Try to cover important logic and edge cases in your tests for better quality.

Use coverage reports to find untested code and add tests there.

Summary

Code coverage measures how much code your tests run.

Use tools like nyc to collect coverage in Node.js projects.

Coverage helps improve test quality and find missing tests.

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