Forms and mutations in Deno

Introduction

Forms let users send data to your app. Mutations change data based on what users send.

When you want users to sign up or log in.

When users fill out a survey or feedback form.

When users update their profile information.

When users submit a comment or post.

When you need to save or change data on the server.

Syntax

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

serve(async (req) => {
  if (req.method === "POST") {
    const formData = await req.formData();
    const name = formData.get("name");
    // perform mutation with name
    return new Response(`Hello, ${name}! Your data was saved.`);
  }
  return new Response(
    `
    <form method="POST">
      <input name="name" placeholder="Your name" required />
      <button type="submit">Send</button>
    </form>
    `,
    { headers: { "content-type": "text/html; charset=utf-8" } }
  );
});

Use req.formData() to get form data from a POST request.

Mutations happen when you change or save data based on form input.

Examples

Extract the email field from the submitted form data.

Deno

const formData = await req.formData();
const email = formData.get("email");

Check if the request is a POST to process form data.

Deno

if (req.method === "POST") {
  // handle form submission
}

Simple HTML form that sends data to the server.

Deno

<form method="POST">
  <input name="message" />
  <button type="submit">Send</button>
</form>

Sample Program

This Deno server shows a form. When you type a message and submit, it sends the message to the server. The server reads the message and replies with a confirmation. This is a simple example of a form and mutation.

Deno

import { serve } from "https://deno.land/std@0.203.0/http/server.ts";

serve(async (req) => {
  if (req.method === "POST") {
    const formData = await req.formData();
    const message = formData.get("message");
    // Simulate saving the message (mutation)
    return new Response(`Message received: ${message}`);
  }

  return new Response(
    `
    <form method="POST">
      <label for="message">Enter message:</label>
      <input id="message" name="message" required />
      <button type="submit">Submit</button>
    </form>
    `,
    { headers: { "content-type": "text/html; charset=utf-8" } }
  );
});

Important Notes

Always use method="POST" for forms that change data.

Use formData.get() to safely get each form field.

Remember to set the response content type to text/html when returning HTML.

Summary

Forms collect user input to send to your app.

Mutations happen when you change or save data from forms.

Deno's req.formData() helps read form data easily.

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