Writing to files (cy.writeFile) in Cypress - Build an Automation Script

Write user data to a JSON file using Cypress

Preconditions (2)

Step 1: Open the Cypress test runner

Step 2: Create a test that uses cy.writeFile to write a JSON object with user details (name, email) to 'cypress/fixtures/user.json'

Step 3: Run the test

Step 4: Verify that the file 'cypress/fixtures/user.json' is created or updated with the correct JSON content

✅ Expected Result: The file 'cypress/fixtures/user.json' contains the exact JSON object with the user details written by the test

Automation Requirements - Cypress

Assertions Needed:

Verify the file 'cypress/fixtures/user.json' exists after writing

Verify the file content matches the JSON object written

Best Practices:

Use cy.writeFile with a valid path relative to the project root

Use JSON.stringify or pass an object directly to cy.writeFile

Avoid hardcoding absolute paths

Use cy.readFile to verify file content

Keep test data simple and clear

Automated Solution

Cypress

describe('Write to file test', () => {
  it('writes user data to a JSON file', () => {
    const userData = {
      name: 'Alice',
      email: '[e-mail]'
    };

    // Write the userData object to a JSON file
    cy.writeFile('cypress/fixtures/user.json', userData);

    // Verify the file content matches userData
    cy.readFile('cypress/fixtures/user.json').should('deep.equal', userData);
  });
});

This test defines a simple user data object with name and email.

It uses cy.writeFile to write this object to cypress/fixtures/user.json. Passing the object directly lets Cypress handle JSON formatting.

Then, cy.readFile reads the file back and asserts that its content deeply equals the original object. This confirms the write was successful and accurate.

Using relative paths keeps the test portable. Assertions ensure the test fails if writing or reading is incorrect.

Common Mistakes - 4 Pitfalls

{'mistake': 'Using an absolute file path in cy.writeFile', 'why_bad': 'Absolute paths can break tests on different machines or environments.', 'correct_approach': "Use relative paths like 'cypress/fixtures/user.json' so the test works anywhere."}

Passing a stringified JSON instead of an object to cy.writeFile

Not verifying the file content after writing

Not waiting for cy.writeFile to complete before reading the file

Bonus Challenge

Now add data-driven testing to write three different user objects to separate JSON files

Show Hint

Practice

1. What is the main purpose of cy.writeFile() in Cypress testing?

easy

A. To delete a file after a test

B. To read data from a file during a test

C. To save data to a file during a test

D. To open a file in the browser

Solution

1. Step 1: Understand the function's role

   cy.writeFile() is used to save or write data into a file during Cypress tests.

2. Step 2: Differentiate from similar functions

   cy.readFile() reads data, but cy.writeFile() writes data. Deleting or opening files is not its purpose.

3. Final Answer:

   To save data to a file during a test -> Option C

4. Quick Check:

   Write data = A [OK]

Hint: Remember: writeFile writes, readFile reads [OK]

Common Mistakes:

• Confusing writeFile with readFile
• Thinking it deletes files
• Assuming it opens files in browser

2. Which of the following is the correct syntax to write the string 'Hello World' to a file named 'greeting.txt' using cy.writeFile()?

easy

A. cy.writeFile('greeting.txt', {text: 'Hello World'})

B. cy.writeFile('greeting.txt', 'Hello World')

C. cy.writeFile('greeting.txt' => 'Hello World')

D. cy.writeFile('greeting.txt', ['Hello World'])

Solution

1. Step 1: Identify correct parameter usage

   cy.writeFile() takes the file path as first argument and the content as second argument, which can be a string.

2. Step 2: Check syntax correctness

   cy.writeFile('greeting.txt', 'Hello World') uses correct syntax: file path string and content string separated by a comma.

3. Final Answer:

   cy.writeFile('greeting.txt', 'Hello World') -> Option B

4. Quick Check:

   Correct syntax uses comma between path and content [OK]

Hint: Use comma between file path and content [OK]

Common Mistakes:

• Using arrow (=>) instead of comma
• Passing content inside an object unnecessarily
• Passing content as an array instead of string

3. Consider the following Cypress code snippet:

cy.writeFile('data.json', { name: 'Alice', age: 30 })
cy.readFile('data.json').then((content) => {
  console.log(content.age)
})

What will be printed in the console?

medium

A. undefined

B. Alice

C. { name: 'Alice', age: 30 }

D. 30

Solution

1. Step 1: Understand what is written to the file

   The object { name: 'Alice', age: 30 } is saved to 'data.json'.

2. Step 2: Understand what is logged

   The code reads the file and logs content.age, which is 30.

3. Final Answer:

   30 -> Option D

4. Quick Check:

   Accessing content.age logs 30 [OK]

Hint: Read file content and access property to get value [OK]

Common Mistakes:

• Expecting the whole object to print
• Confusing name and age values
• Assuming undefined because of async

4. You wrote this code to save JSON data:

cy.writeFile('user.json', '{"name": "Bob", "age": 25}')

But when you read the file, you get a string instead of an object. What is the problem?

medium

A. The JSON data is passed as a string, not as an object

B. The file path is incorrect

C. cy.writeFile cannot write JSON data

D. The file extension must be .txt

Solution

1. Step 1: Analyze the data argument

   The data is passed as a JSON string, not as a JavaScript object.

2. Step 2: Understand how cy.writeFile handles data

   If you pass a string, it writes it as a string. To write JSON, pass an object directly.

3. Final Answer:

   The JSON data is passed as a string, not as an object -> Option A

4. Quick Check:

   Pass object, not string, to write JSON [OK]

Hint: Pass object, not JSON string, to write JSON files [OK]

Common Mistakes:

• Passing JSON as string instead of object
• Blaming file path without checking data
• Thinking file extension affects JSON writing

5. You want to append a new line 'Test passed' to an existing file 'results.txt' using cy.writeFile(). Which code snippet correctly appends the text without overwriting the file?

hard

A. cy.writeFile('results.txt', 'Test passed', { flag: 'a+' })

B. cy.writeFile('results.txt', 'Test passed')

C. cy.writeFile('results.txt', 'Test passed', { overwrite: false })

D. cy.writeFile('results.txt', 'Test passed', { append: true })

Solution

1. Step 1: Understand file writing flags

   To append text, the file must be opened with append flag, which is 'a+' in Node.js.

2. Step 2: Check Cypress options for cy.writeFile

   Cypress supports passing { flag: 'a+' } to append instead of overwrite.

3. Final Answer:

   cy.writeFile('results.txt', 'Test passed', { flag: 'a+' }) -> Option A

4. Quick Check:

   Use flag 'a+' to append text [OK]

Hint: Use { flag: 'a+' } option to append text [OK]

Common Mistakes:

• Omitting flag option causes overwrite
• Using non-existent options like append: true
• Assuming overwrite: false appends text