Why Code coverage plugin in Cypress? - Purpose & Use Cases

The Big Idea

What if you could instantly see which parts of your app are still untested and risky?

The Scenario

Imagine testing a web app by clicking through every page and feature manually, hoping you didn't miss any part of the code.

You write down what you tested on paper or a simple list, but you can't be sure if every line of code was actually checked.

The Problem

Manual testing is slow and tiring. You might forget some features or miss hidden bugs.

Without clear proof, you can't tell if your tests cover all the important parts of your app.

This leads to wasted time and risky releases.

The Solution

The code coverage plugin automatically tracks which parts of your code run during tests.

It shows you exactly what is tested and what is not, so you can focus on missing areas.

This saves time, reduces errors, and builds confidence in your testing.

Before vs After

✗ Before

Run tests manually and guess coverage based on memory or notes.

✓ After

Use a code coverage plugin to see a report of tested and untested code automatically.

What It Enables

It enables you to confidently deliver high-quality software by knowing exactly how much of your code is tested.

Real Life Example

A developer runs Cypress tests with the code coverage plugin and finds that some critical functions are never tested.

They add tests for those parts, preventing bugs before release.

Key Takeaways

Manual testing can miss important code areas.

Code coverage plugin shows exactly what code is tested.

This helps improve test quality and software reliability.

Practice

1. What is the main purpose of the Cypress code coverage plugin?

easy

A. To show which parts of your code are tested and which are not

B. To speed up test execution time

C. To automatically fix bugs in your code

D. To generate user interface screenshots

Solution

1. Step 1: Understand code coverage concept

   Code coverage helps identify which lines or branches of code are executed during tests.

2. Step 2: Match purpose with plugin function

   The Cypress code coverage plugin tracks and reports tested vs untested code parts.

3. Final Answer:

   To show which parts of your code are tested and which are not -> Option A

4. Quick Check:

   Code coverage = tested vs untested code [OK]

Hint: Code coverage = shows tested and missed code parts [OK]

Common Mistakes:

• Thinking it speeds up tests
• Believing it fixes bugs automatically
• Confusing it with screenshot tools

2. Which file should you modify to enable the Cypress code coverage plugin support?

easy

A. cypress/support/e2e.js

B. cypress/fixtures/data.json

C. package.json

D. cypress.config.js

Solution

1. Step 1: Identify where plugin support code goes

   The plugin support code is added in the support file to load before tests run.

2. Step 2: Choose correct support file path

   By default, Cypress uses cypress/support/e2e.js for test support setup.

3. Final Answer:

   cypress/support/e2e.js -> Option A

4. Quick Check:

   Support file = cypress/support/e2e.js [OK]

Hint: Support code goes in cypress/support/e2e.js [OK]

Common Mistakes:

• Modifying cypress.config.js only
• Changing package.json for plugin support
• Editing fixture files for plugin setup

3. Given this snippet in cypress/support/e2e.js:

import '@cypress/code-coverage/support';

What will happen when you run your tests?

medium

A. Tests will fail due to missing plugin installation

B. No effect; code coverage requires config changes only

C. Code coverage data will be collected during tests

D. Tests will run but coverage report will be empty

Solution

1. Step 1: Understand import statement effect

   Importing '@cypress/code-coverage/support' enables coverage collection hooks.

2. Step 2: Recognize plugin behavior during test run

   This import activates coverage tracking so data is gathered as tests run.

3. Final Answer:

   Code coverage data will be collected during tests -> Option C

4. Quick Check:

   Import support = coverage data collected [OK]

Hint: Importing support enables coverage collection [OK]

Common Mistakes:

• Assuming import alone fails tests
• Thinking config changes alone enable coverage
• Believing coverage report is empty without extra steps

4. You added the code coverage plugin but your coverage report is empty after tests run. What is the most likely cause?

medium

A. You did not write any tests

B. You forgot to instrument your app code with the coverage tool

C. You imported the plugin support file twice

D. You ran tests in headless mode

Solution

1. Step 1: Understand coverage instrumentation

   Coverage requires your app code to be instrumented to track executed lines.

2. Step 2: Identify missing instrumentation effect

   Without instrumentation, coverage plugin cannot detect which code ran, so report is empty.

3. Final Answer:

   You forgot to instrument your app code with the coverage tool -> Option B

4. Quick Check:

   Missing instrumentation = empty coverage report [OK]

Hint: Instrument app code to get coverage data [OK]

Common Mistakes:

• Assuming no tests means empty report
• Thinking duplicate imports cause empty report
• Believing headless mode disables coverage

5. You want to improve your test suite by using the Cypress code coverage plugin. Which steps should you follow to get a working coverage report? Select the correct sequence.

hard

A. Instrument app code, run tests, install plugin, import support in e2e.js, check report

B. Run tests, install plugin, import support in e2e.js, instrument app code, check report

C. Import support in e2e.js, run tests, install plugin, instrument app code, check report

D. Install plugin, instrument app code, import support in e2e.js, run tests, check report

Solution

1. Step 1: Install the code coverage plugin

   First, add the plugin package to your project dependencies.

2. Step 2: Instrument your application code

   Modify your app build to include coverage instrumentation for tracking.

3. Step 3: Import the plugin support file in cypress/support/e2e.js

   This enables coverage hooks during test execution.

4. Step 4: Run your Cypress tests

   Tests will now collect coverage data as they execute.

5. Step 5: Check the generated coverage report

   Review which code was covered and improve tests accordingly.

6. Final Answer:

   Install plugin, instrument app code, import support in e2e.js, run tests, check report -> Option D

7. Quick Check:

   Correct setup sequence = Install plugin, instrument app code, import support in e2e.js, run tests, check report [OK]

Hint: Install, instrument, import support, run tests, check report [OK]

Common Mistakes:

• Running tests before installing plugin
• Skipping instrumentation step
• Importing support after running tests