Code coverage basics in Node.js - Mini Project: Build & Apply

Code coverage basics

📖 Scenario: You are working on a small Node.js project. You want to check how much of your code is tested by your tests. This helps you find parts of your code that need more testing.

🎯 Goal: Set up a simple Node.js function and a test file. Then configure a code coverage tool to measure how much of your function is tested.

📋 What You'll Learn

Create a Node.js function in a file called math.js

Create a test file called math.test.js that tests the function

Add a configuration to enable code coverage using nyc

Run the tests with coverage and verify the coverage report

💡 Why This Matters

🌍 Real World

Code coverage is used in real projects to check how much code is tested. This helps developers find untested parts and improve test quality.

💼 Career

Many software jobs require writing tests and using coverage tools like nyc to ensure code quality and reliability.

1

Create the function to test

Create a file called math.js and write a function named add that takes two parameters a and b and returns their sum.

Node.js

// Your code here

Hint

Define a function named add with parameters a and b. Return their sum. Export the function using module.exports.

2

Write a test for the function

Create a file called math.test.js. Import the add function from math.js. Write a test using test that checks if add(2, 3) returns 5.

Node.js

const { add } = require('./math');

// Your test code here

Hint

Import add from math.js. Use test to check if add(2, 3) equals 5.

3

Configure code coverage with nyc

Create a file called .nycrc in your project root. Add JSON content with { "all": true } to enable coverage for all files. Also, add a script in package.json called test:coverage that runs nyc --reporter=lcov --reporter=text npm test.

Node.js

{
  "all": true
}

// Add the following script to package.json:
// "test:coverage": "nyc --reporter=lcov --reporter=text npm test"

Hint

Create a .nycrc file with { "all": true }. Add a test:coverage script in package.json to run tests with coverage using nyc.

4

Run tests with coverage and check report

Run the command npm run test:coverage in your terminal. This runs your tests and shows a coverage report in text format. Verify that the coverage report shows 100% coverage for statements, branches, functions, and lines.

Node.js

// Run the command below in your terminal
// npm run test:coverage

// Check the coverage report output

Hint

Run npm run test:coverage in your terminal. Look for the coverage summary showing 100% coverage.

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