Docker execution in Cypress - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Cypress Docker container test run output

You run a Cypress test inside a Docker container using the command:

docker run -it -v $PWD:/e2e -w /e2e cypress/included:12.0.0

The test suite contains 3 passing tests and 1 failing test. What will the Docker container output indicate after the run?

A The container exits with code 0 and shows '4 tests passed' in the logs.

B The container exits with a non-zero code but shows 'All tests passed' in the logs.

C The container exits with code 0 but shows '1 test failed' in the logs.

D The container exits with a non-zero code and shows '3 tests passed, 1 test failed' in the logs.

💡 Hint

Remember that Cypress Docker images exit with a non-zero code if any test fails.

When Cypress tests run inside the official Docker container, if any test fails, the container exits with a non-zero code. The logs will show the number of passing and failing tests accurately.

❓ assertion

intermediate

Correct assertion to verify test run success in Cypress Docker

You want to write a script that runs Cypress tests inside Docker and asserts the container exited successfully (all tests passed). Which assertion correctly checks the Docker container exit code in a Node.js script?

Cypress

const { exec } = require('child_process');
exec('docker run -it -v $PWD:/e2e -w /e2e cypress/included:12.0.0', (error, stdout, stderr) => {
  // Which assertion below correctly verifies success?
});

A assert.strictEqual(error, null);

B assert.strictEqual(error.code, 0);

C assert.strictEqual(error, 0);

D assert.strictEqual(error, undefined);

💡 Hint

In Node.js exec callback, error is null if the command succeeds.

The exec callback's error argument is null when the command exits with code 0 (success). Checking error.code or error directly is incorrect because error is either null or an Error object.

🔧 Debug

advanced

Debugging Cypress tests failing silently in Docker

You run Cypress tests inside Docker, but the container exits with code 0 and the logs show no test results. What is the most likely cause?

A The Docker container ran out of memory and crashed silently.

B The test files are not mounted correctly into the container volume.

C The Cypress version in Docker is incompatible with the test code.

D The Docker network settings prevent Cypress from running tests.

💡 Hint

Check if your test files are accessible inside the container.

If the test files are not mounted or mapped correctly into the Docker container, Cypress will run but find no tests, exiting with code 0 and no results.

❓ framework

advanced

Best practice for running Cypress tests in CI with Docker

Which approach is best to run Cypress tests inside Docker in a Continuous Integration (CI) pipeline to ensure consistent test results?

A Use the official Cypress Docker image with a fixed version tag and mount the test code as a volume.

B Build a custom Docker image with the latest Cypress version installed dynamically at runtime.

C Run Cypress tests directly on the host machine without Docker for faster execution.

D Use the official Cypress Docker image without specifying a version tag to always get the latest.

💡 Hint

Consistency and reproducibility are key in CI environments.

Using the official Cypress Docker image with a fixed version tag ensures tests run in a consistent environment. Mounting the test code as a volume allows easy updates without rebuilding the image.

🧠 Conceptual

expert

Understanding Cypress Docker container exit codes

What does a non-zero exit code from a Cypress Docker container indicate after test execution?

A The tests were skipped because no test files were found.

B All tests passed successfully without any errors.

C At least one test failed or an error occurred during test execution.

D The Docker container failed to start due to configuration issues.

💡 Hint

Non-zero exit codes usually signal failure or errors.

Cypress Docker containers exit with code 0 only if all tests pass. A non-zero exit code means one or more tests failed or there was an error during execution.

Practice

1. What is the main benefit of running Cypress tests inside a Docker container?

easy

A. It provides a clean and consistent environment for tests.

B. It makes tests run faster on all machines.

C. It automatically fixes test failures.

D. It removes the need for writing test code.

Solution

1. Step 1: Understand Docker's role in testing

   Docker creates isolated environments that are the same everywhere.

2. Step 2: Connect Docker benefits to Cypress tests

   Running Cypress in Docker ensures tests run in a clean, consistent setup, avoiding environment differences.

3. Final Answer:

   It provides a clean and consistent environment for tests. -> Option A

4. Quick Check:

   Docker = consistent test environment [OK]

Hint: Docker ensures same environment everywhere for tests [OK]

Common Mistakes:

• Thinking Docker speeds up tests automatically
• Believing Docker fixes test code errors
• Assuming Docker removes need for tests

2. Which Docker command correctly runs Cypress tests mounting the current folder as a volume?

easy

A. docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0

B. docker run -p 8080:80 cypress/included:12.0.0

C. docker build -t cypress-test .

D. docker run cypress/included:12.0.0 --no-volume

Solution

1. Step 1: Identify volume mounting syntax

   The -v option mounts a folder; $(pwd):/e2e mounts current folder to /e2e in container.

2. Step 2: Check command correctness

   docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 uses -v and -w to set working directory, which is correct for running Cypress tests.

3. Final Answer:

   docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 -> Option A

4. Quick Check:

   Volume mount = -v [OK]

Hint: Use -v to mount volumes in Docker run command [OK]

Common Mistakes:

• Using -p instead of -v for volume mounting
• Forgetting to set working directory with -w
• Confusing docker build with docker run

3. Given this Docker command:

docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0

What will happen if your current folder has no Cypress tests?

medium

A. Docker will throw a syntax error and stop.

B. The container will fail to start due to missing tests.

C. Cypress will automatically create sample tests.

D. The container will run but no tests will execute.

Solution

1. Step 1: Understand volume mounting effect

   The current folder is mounted inside the container at /e2e, so container sees the files you have.

2. Step 2: Consider what happens with no tests

   If no test files exist, Cypress runs but finds nothing to execute, so no tests run but container runs fine.

3. Final Answer:

   The container will run but no tests will execute. -> Option D

4. Quick Check:

   No tests = no execution, container still runs [OK]

Hint: No tests means no runs, but container still starts [OK]

Common Mistakes:

• Expecting Docker syntax error for missing files
• Thinking Cypress auto-generates tests
• Assuming container fails to start without tests

4. You run Cypress tests in Docker but see this error:
ERROR: Cannot find module 'cypress'
What is the most likely cause?

medium

A. You forgot to mount your test folder with node_modules.

B. You used the wrong Docker image without Cypress installed.

C. Your Dockerfile syntax is incorrect.

D. Your tests have syntax errors.

Solution

1. Step 1: Analyze the error message

   The error says Cypress module is missing, meaning Cypress is not installed in the container.

2. Step 2: Check Docker image used

   If you use a Docker image without Cypress pre-installed, this error occurs. Using cypress/included images avoids this.

3. Final Answer:

   You used the wrong Docker image without Cypress installed. -> Option B

4. Quick Check:

   Missing module = wrong image [OK]

Hint: Use official cypress/included image to avoid missing Cypress [OK]

Common Mistakes:

• Blaming test syntax errors for missing module
• Assuming volume mount fixes missing Cypress
• Confusing Dockerfile syntax with missing module

5. You want to run Cypress tests in Docker and save screenshots and videos outside the container. Which command correctly achieves this?

hard

A. docker run -p 8080:80 -v $(pwd):/e2e cypress/included:12.0.0

B. docker run -v $(pwd):/e2e -w /e2e cypress/included:12.0.0 --save-artifacts

C. docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0

D. docker run --rm cypress/included:12.0.0

Solution

1. Step 1: Understand volume mounting for artifacts

   To save screenshots and videos outside Docker, mount their folders as volumes from host to container.

2. Step 2: Check command for multiple volume mounts

   docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0 mounts the whole project plus screenshots and videos folders separately, ensuring artifacts persist on host.

3. Final Answer:

   docker run -v $(pwd):/e2e -v $(pwd)/cypress/screenshots:/e2e/cypress/screenshots -v $(pwd)/cypress/videos:/e2e/cypress/videos -w /e2e cypress/included:12.0.0 -> Option C

4. Quick Check:

   Mount artifact folders to save outside container [OK]

Hint: Mount screenshots and videos folders as volumes [OK]

Common Mistakes:

• Not mounting screenshots/videos folders separately
• Using -p port mapping instead of volume mounts
• Assuming --save-artifacts flag exists