Forms and mutations in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a form mutation in web development?

A form mutation is a change or update made to data on the server when a user submits a form, such as creating, updating, or deleting information.

beginner

How do you handle form data submission in Deno?

In Deno, you handle form data by reading the request body, parsing the form data, and then performing the desired mutation like saving or updating data on the server.

intermediate

Why is it important to validate form data before mutation?

Validating form data ensures the data is correct and safe before changing the server state, preventing errors and security issues like invalid input or injection attacks.

beginner

What HTTP method is commonly used for form mutations and why?

POST is commonly used because it sends data to the server to create or update resources, unlike GET which only retrieves data.

beginner

How can you provide feedback to users after a form mutation in Deno?

You can send a response with a success or error message after processing the form, which the frontend can display to inform the user about the result.

Which HTTP method is best suited for submitting form data that changes server data?

A DELETE

B GET

C POST

D OPTIONS

POST is used to send data to the server to create or update resources, making it ideal for form mutations.

In Deno, how do you access form data sent in a POST request?

A Using request.body() and parsing form data

B Using URL parameters

C Using cookies

D Using request.headers only

You read the request body and parse it to get form data in Deno.

Why should you validate form data before performing mutations?

A To ensure data is correct and safe

B To speed up the server

C To avoid using POST method

D To prevent users from submitting forms

Validation prevents errors and security risks by checking data before changing server state.

What is a common way to inform users about the result of a form submission?

A Reload the page without message

B Use GET method instead

C Ignore the submission

D Send a response message after mutation

Sending a response message helps users know if their action succeeded or failed.

Which of these is NOT a typical form mutation action?

A Deleting data

B Retrieving data

C Creating data

D Updating data

Retrieving data is usually a read operation, not a mutation.

Explain how you would handle a form submission in Deno to update user information.

Think about the steps from receiving data to confirming the update.

Describe why validating form data is important before performing mutations on the server.

Consider what could happen if bad data is accepted.

Practice

1. What does the req.formData() method do in a Deno server context?

easy

A. It reads and returns the submitted form data from the request.

B. It sends form data back to the client.

C. It validates the form data automatically.

D. It clears the form data from the request.

Solution

1. Step 1: Understand the purpose of req.formData()

   This method is designed to extract form data sent by the client in a POST request.

2. Step 2: Identify what the method returns

   It returns a FormData object containing key-value pairs from the submitted form.

3. Final Answer:

   It reads and returns the submitted form data from the request. -> Option A

4. Quick Check:

   Form data reading = It reads and returns the submitted form data from the request. [OK]

Hint: Remember: formData() reads input, it doesn't send or clear it [OK]

Common Mistakes:

• Thinking formData() sends data to client
• Assuming formData() validates data automatically
• Believing formData() clears data from request

2. Which of the following is the correct way to asynchronously get form data from a request in Deno?

easy

A. const data = await req.formData();

B. const data = req.getFormData();

C. const data = req.formData();

D. const data = await req.getFormData();

Solution

1. Step 1: Recall the method signature for formData()

   In Deno, req.formData() returns a Promise, so it must be awaited.

2. Step 2: Identify the correct syntax

   The correct syntax uses await req.formData(); to get the form data asynchronously.

3. Final Answer:

   const data = await req.formData(); -> Option A

4. Quick Check:

   Async form data retrieval = const data = await req.formData(); [OK]

Hint: Always await formData() since it returns a Promise [OK]

Common Mistakes:

• Forgetting to use await with formData()
• Using a non-existent method getFormData()
• Calling formData() without async context

3. Given this Deno server code snippet handling a POST request:

const formData = await req.formData();
const name = formData.get('name');
return new Response(`Hello, ${name}!`);

What will be the response if the form submitted has name=Alice?

medium

A. Hello, !

B. Hello, undefined!

C. Hello, null!

D. Hello, Alice!

Solution

1. Step 1: Extract the 'name' value from formData

   The code uses formData.get('name') which returns the string 'Alice' from the submitted form.

2. Step 2: Construct the response string

   The response template string inserts 'Alice' correctly, resulting in 'Hello, Alice!'.

3. Final Answer:

   Hello, Alice! -> Option D

4. Quick Check:

   formData.get('name') = 'Alice' [OK]

Hint: formData.get('key') returns the submitted value or null if missing [OK]

Common Mistakes:

• Assuming get() returns undefined instead of null
• Not using await with formData() causing errors
• Misreading template string interpolation

4. Identify the error in this Deno code snippet handling form submission:

const formData = req.formData();
const email = formData.get('email');
return new Response(email);

medium

A. Response cannot return a string directly.

B. formData.get() is not a valid method on FormData.

C. Missing await before req.formData() causing a Promise error.

D. The variable email is not declared.

Solution

1. Step 1: Check how formData() is called

   The method req.formData() returns a Promise and must be awaited.

2. Step 2: Identify the error caused by missing await

   Without await, formData is a Promise, so calling get() on it causes a runtime error.

3. Final Answer:

   Missing await before req.formData() causing a Promise error. -> Option C

4. Quick Check:

   Always await formData() to get FormData object [OK]

Hint: Always await async calls like formData() before using results [OK]

Common Mistakes:

• Calling get() on a Promise instead of FormData
• Assuming formData() is synchronous
• Ignoring async/await syntax rules

5. You want to update a user's profile using a form in Deno. The form sends username and age. Which code correctly reads the form and updates a user object, ensuring age is stored as a number?

hard

A. const formData = await req.formData(); const user = { username: formData.get('username'), age: formData.get('age') };

B. const formData = await req.formData(); const user = { username: formData.get('username'), age: Number(formData.get('age')) };

C. const formData = req.formData(); const user = { username: formData.get('username'), age: parseInt(formData.get('age')) };

D. const formData = await req.formData(); const user = { username: formData.get('username'), age: parseFloat(formData.get('username')) };

Solution

1. Step 1: Await formData() to get the form data object

   Using await req.formData() correctly retrieves the form data asynchronously.

2. Step 2: Convert the 'age' string to a number properly

   Using Number(formData.get('age')) converts the age string to a number safely.

3. Step 3: Check the username assignment

   Assigning username: formData.get('username') correctly stores the username string.

4. Final Answer:

   const formData = await req.formData(); const user = { username: formData.get('username'), age: Number(formData.get('age')) }; -> Option B

5. Quick Check:

   Await + Number conversion = const formData = await req.formData(); const user = { username: formData.get('username'), age: Number(formData.get('age')) }; [OK]

Hint: Always await formData() and convert numeric strings with Number() [OK]

Common Mistakes:

• Not awaiting formData() causing errors
• Storing age as string without conversion
• Parsing wrong field for age