Code coverage basics in Node.js - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is code coverage?

Code coverage measures how much of your code is tested by automated tests. It shows which parts of the code ran during testing.

beginner

Name three common types of code coverage.

1. Line coverage: checks if each line of code ran.
2. Branch coverage: checks if each decision path ran.
3. Function coverage: checks if each function was called.

intermediate

Why is 100% code coverage not always necessary?

Because some code is hard to test or not critical. Also, high coverage doesn't guarantee no bugs, but it helps find many issues.

beginner

How do you generate code coverage reports in Node.js?

You can use tools like 'nyc' (Istanbul) with your test runner. Run tests with coverage enabled to see which code ran.

beginner

What does a coverage report typically show?

It shows percentages of lines, branches, and functions covered. It highlights uncovered code to improve tests.

What does line coverage measure?

A If the code compiles without errors

B If each function was called

C If each line of code was executed during tests

D If each branch decision was tested

Line coverage checks if every line of code ran during testing.

Which tool is commonly used in Node.js for code coverage?

A Jest

B nyc (Istanbul)

C Mocha

D Express

'nyc' is a popular code coverage tool for Node.js, often used with test runners.

Why might 100% code coverage be unrealistic?

A Because some code is unreachable or hard to test

B Because tests are not important

C Because coverage tools are inaccurate

D Because code coverage slows down the app

Some code paths are difficult or unnecessary to test, so 100% coverage is not always practical.

What does branch coverage check?

A If each function was called

B If the code is formatted correctly

C If the code runs without errors

D If each decision path (like if/else) was tested

Branch coverage ensures all decision paths in the code are tested.

What is the main benefit of using code coverage?

A To find untested parts of code and improve test quality

B To make the code run faster

C To reduce the size of the code

D To automatically fix bugs

Code coverage helps identify which parts of code lack tests, improving overall test quality.

Explain what code coverage is and why it is useful in testing Node.js applications.

Think about how you check if your tests run all parts of your code.

Describe how you would set up code coverage reporting in a Node.js project.

Consider the tools and commands you use to see which code your tests cover.

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