Custom plugin development in Cypress - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom plugin in Cypress?

A custom plugin in Cypress is a piece of code that extends Cypress's functionality by adding new tasks or modifying existing behavior, allowing you to customize test runs.

beginner

Where do you define custom plugins in a Cypress project?

Custom plugins are defined in the cypress/plugins/index.js file, where you can register new tasks or modify Cypress events.

intermediate

How do you register a new task in a Cypress plugin?

You use the on('task', { ... }) event handler inside the plugin file to register new tasks that can be called from test code.

intermediate

Why use custom plugins instead of writing all code in test files?

Custom plugins allow you to run Node.js code outside the browser context, access system resources, or perform complex operations that are not possible directly in test files.

advanced

What is the role of the on and config parameters in the plugin function?

The on parameter lets you listen to Cypress events like task, while config provides the current Cypress configuration that you can modify or use.

Where do you add custom plugin code in a Cypress project?

A cypress/integration/tests.js

B cypress/support/commands.js

C cypress/plugins/index.js

D package.json

Custom plugins are added in cypress/plugins/index.js to extend Cypress functionality.

Which Cypress event is used to register custom tasks in a plugin?

A on('window:load', ...)

B on('beforeEach', ...)

C on('test:after:run', ...)

D on('task', ...)

The 'task' event is used to register custom tasks that run in the Node environment.

Why might you create a custom plugin instead of writing code directly in tests?

A To make tests run faster

B To run Node.js code outside the browser

C To avoid writing assertions

D To skip tests automatically

Custom plugins allow running Node.js code that cannot run in the browser context of tests.

What does the config parameter in the plugin function provide?

A Current Cypress configuration settings

B Test case data

C Browser console logs

D Test execution results

The config parameter gives access to Cypress configuration which can be read or modified.

Which file would you NOT modify to create a custom Cypress plugin?

A cypress/integration/sample_spec.js

B cypress/support/index.js

C cypress/plugins/index.js

D cypress/plugins/customPlugin.js

Test files like integration/sample_spec.js are for writing tests, not plugins.

Explain how to create and register a custom task in a Cypress plugin.

Think about the plugin file and how Cypress listens for tasks.

Describe why and when you would use a custom plugin in Cypress testing.

Consider what test code cannot do alone.

Practice

1. What is the main purpose of creating a custom plugin in Cypress?

easy

A. To add new commands or tasks that extend Cypress functionality

B. To change the browser's default behavior during tests

C. To write test cases in a different programming language

D. To speed up the execution of all tests automatically

Solution

1. Step 1: Understand what plugins do in Cypress

   Plugins allow adding new commands or tasks that Cypress does not provide by default.

2. Step 2: Differentiate from other options

   Changing browser behavior or writing tests in other languages is not the role of plugins. Speeding up tests automatically is not guaranteed by plugins.

3. Final Answer:

   To add new commands or tasks that extend Cypress functionality -> Option A

4. Quick Check:

   Plugins extend Cypress features = D [OK]

Hint: Plugins add new commands or tasks to Cypress [OK]

Common Mistakes:

• Thinking plugins change browser behavior directly
• Confusing plugins with test case languages
• Assuming plugins automatically speed up tests

2. Which file is typically used to define custom plugins in a Cypress project?

easy

A. cypress/plugins/index.js

B. cypress/support/commands.js

C. cypress/integration/spec.js

D. cypress.json

Solution

1. Step 1: Identify the plugin file location

   Custom plugins are defined in the cypress/plugins/index.js file where Node.js code runs.

2. Step 2: Differentiate from support and config files

   support/commands.js is for custom commands in browser tests, integration/spec.js is for test specs, and cypress.json is config.

3. Final Answer:

   cypress/plugins/index.js -> Option A

4. Quick Check:

   Plugins file = C [OK]

Hint: Plugins go in cypress/plugins/index.js file [OK]

Common Mistakes:

• Confusing plugins file with support commands file
• Choosing test spec files for plugins
• Selecting config file for plugin code

3. Given this plugin code snippet in cypress/plugins/index.js:

module.exports = (on, config) => {
  on('task', {
    logMessage(message) {
      console.log(message)
      return null
    }
  })
}

What happens when the test calls cy.task('logMessage', 'Hello')?

medium

A. The test fails because 'logMessage' is not a valid Cypress command

B. The message 'Hello' is printed in the browser console, and the task returns undefined

C. The message 'Hello' is printed in the Node.js console, and the task returns null

D. Nothing happens because tasks cannot receive arguments

Solution

1. Step 1: Understand the plugin task definition

   The plugin defines a task named 'logMessage' that logs the message to the Node.js console and returns null.

2. Step 2: Understand cy.task behavior

   Calling cy.task('logMessage', 'Hello') sends 'Hello' to the plugin task, which logs it in Node.js, not browser, and returns null.

3. Final Answer:

   The message 'Hello' is printed in the Node.js console, and the task returns null -> Option C

4. Quick Check:

   cy.task logs in Node.js console = A [OK]

Hint: cy.task runs plugin code in Node.js, not browser console [OK]

Common Mistakes:

• Thinking console.log prints in browser console
• Assuming tasks cannot take arguments
• Confusing tasks with custom commands

4. Identify the error in this plugin code snippet:

module.exports = (on, config) => {
  on('task', {
    multiplyNumbers(a, b) {
      return a * b
    }
  })
}

And select the correct fix.

medium

A. No error, the code is correct as is

B. Change the task to accept a single argument object like {a, b} instead of two arguments

C. Replace 'on' with 'off' to register the task properly

D. Return a Promise instead of a direct value

Solution

1. Step 1: Understand how Cypress tasks receive arguments

   Cypress tasks accept only one argument. Multiple arguments must be wrapped in an object.

2. Step 2: Identify the error and fix

   The function multiplyNumbers expects two arguments, but Cypress will pass only one. Fix by accepting a single object with properties a and b.

3. Final Answer:

   Change the task to accept a single argument object like {a, b} instead of two arguments -> Option B

4. Quick Check:

   Tasks accept one argument object = B [OK]

Hint: Tasks get one argument; use object to pass multiple values [OK]

Common Mistakes:

• Defining task functions with multiple parameters
• Using 'off' instead of 'on' to register tasks
• Thinking tasks must return Promises always

5. You want to create a custom plugin that reads a JSON file from disk and returns its content to your test. Which approach correctly implements this plugin task?

hard

A. Use console.log to print file content and return undefined

B. Use cy.readFile inside the plugin task to read the file asynchronously

C. Use browser fetch API inside the plugin task to get the file content

D. Use Node.js fs.readFileSync inside the plugin task and return parsed JSON

Solution

1. Step 1: Understand plugin environment

   Plugins run in Node.js, so Node.js modules like fs can be used to read files synchronously or asynchronously.

2. Step 2: Choose correct method to read file

   cy.readFile is a test command, not available in plugins. Browser fetch is not available in Node.js plugin context. Using fs.readFileSync and returning parsed JSON is correct.

3. Final Answer:

   Use Node.js fs.readFileSync inside the plugin task and return parsed JSON -> Option D

4. Quick Check:

   Plugins use Node.js fs to read files = A [OK]

Hint: Use Node.js fs module in plugins, not cy commands [OK]

Common Mistakes:

• Trying to use cy commands inside plugins
• Using browser APIs in Node.js plugin code
• Returning undefined instead of file content