Form validation in Svelte - Mini Project: Build & Apply

Form validation

📖 Scenario: You are building a simple user registration form on a website. The form needs to check that the user enters their name and email correctly before submitting.

🎯 Goal: Create a Svelte component that has a form with name and email input fields. Add validation so the form shows error messages if the name is empty or the email is not valid. The submit button should only be enabled when the form is valid.

📋 What You'll Learn

Create name and email variables to hold input values

Add a validEmail regular expression to check email format

Write a function validateForm that returns true only if name is not empty and email matches validEmail

Bind inputs to variables and disable submit button if validateForm() returns false

Show error messages below inputs when invalid

💡 Why This Matters

🌍 Real World

Forms are everywhere on websites and apps. Validating user input helps prevent errors and improves user experience.

💼 Career

Knowing how to build forms with validation is a key skill for frontend developers working with modern frameworks like Svelte.

1

Set up form data variables

Create two variables called name and email and set both to empty strings.

Svelte

# Your code here

Hint

Use let to create variables in Svelte.

2

Add email validation pattern

Add a variable called validEmail and set it to the regular expression /^\S+@\S+\.\S+$/ to check email format.

Svelte

let name = "";
let email = "";
# Your code here

Hint

Use const for the regular expression since it won't change.

3

Create form validation function

Write a function called validateForm that returns true only if name is not empty and email matches validEmail. Otherwise, return false.

Svelte

let name = "";
let email = "";
const validEmail = /^\S+@\S+\.\S+$/;

# Your code here

Hint

Use name.trim() !== "" to check name is not empty and validEmail.test(email) to check email.

4

Build form with validation feedback

Create a form with two inputs bound to name and email. Add error messages below each input that show if the input is invalid. Disable the submit button if validateForm() returns false.

Svelte

let name = "";
let email = "";
const validEmail = /^\S+@\S+\.\S+$/;

function validateForm() {
    return name.trim() !== "" && validEmail.test(email);
}

Hint

Use Svelte's bind:value to connect inputs to variables. Use {#if} blocks to show errors. Disable the button with disabled={!validateForm()}.

Practice

1. What is the main purpose of form validation in a Svelte application?

easy

A. To check user input before sending data

B. To style the form elements

C. To load data from the server

D. To create new components

Solution

1. Step 1: Understand form validation role

   Form validation ensures user input is correct before submission.

2. Step 2: Identify the correct purpose

   Checking input prevents errors and improves user experience.

3. Final Answer:

   To check user input before sending data -> Option A

4. Quick Check:

   Form validation = check input [OK]

Hint: Validation means checking input before sending [OK]

Common Mistakes:

• Confusing validation with styling
• Thinking validation loads data
• Mixing validation with component creation

2. Which Svelte syntax correctly binds an input's value for validation?

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall Svelte binding syntax

   Svelte uses bind:value={variable} to bind input values.

2. Step 2: Identify correct syntax

   <input bind:value={name} /> uses correct curly braces and binding keyword.

3. Final Answer:

   <input bind:value={name} /> -> Option B

4. Quick Check:

   bind:value with braces = correct [OK]

Hint: Use bind:value with curly braces for binding [OK]

Common Mistakes:

• Omitting curly braces
• Using colon instead of equals
• Using value= instead of bind:value=

3. What will be the output when submitting this Svelte form if the input is empty?

<script>
  let email = '';
  let error = '';
  function validate() {
    if (!email) {
      error = 'Email is required';
    } else {
      error = '';
    }
  }
</script>

<form on:submit|preventDefault={validate}>
  <input bind:value={email} type="email" />
  <button type="submit">Submit</button>
  <p>{error}</p>
</form>

medium

A. Error message 'Email is required' is shown

B. Form submits with no error message

C. Syntax error prevents form rendering

D. Form reloads the page

Solution

1. Step 1: Analyze validation function

   If email is empty, error is set to 'Email is required'.

2. Step 2: Check form submission behavior

   Submit event is prevented from default, so page does not reload, and error message shows.

3. Final Answer:

   Error message 'Email is required' is shown -> Option A

4. Quick Check:

   Empty input triggers error message [OK]

Hint: Empty input triggers error message display [OK]

Common Mistakes:

• Ignoring preventDefault causing page reload
• Expecting form to submit without error
• Confusing syntax error with logic error

4. Identify the error in this Svelte form validation code:

<script>
  let password = '';
  let error = '';
  function check() {
    if(password.length < 6) {
      error = 'Password too short';
    }
  }
</script>

<form on:submit={check}>
  <input bind:value={password} type="password" />
  <button type="submit">Login</button>
  <p>{error}</p>
</form>

medium

A. Incorrect binding syntax for password

B. Error message variable not declared

C. Missing preventDefault on submit event

D. Password length check uses wrong operator

Solution

1. Step 1: Review submit event handling

   The form's submit event calls check but does not prevent default, so page reloads.

2. Step 2: Identify missing preventDefault

   Without preventDefault, error message disappears on reload, breaking validation UX.

3. Final Answer:

   Missing preventDefault on submit event -> Option C

4. Quick Check:

   Submit needs preventDefault to stop reload [OK]

Hint: Always use preventDefault on form submit to keep validation visible [OK]

Common Mistakes:

• Forgetting preventDefault causes page reload
• Thinking binding syntax is wrong when it is correct
• Assuming error variable is missing

5. How can you improve accessibility when showing validation errors in a Svelte form?

hard

A. Remove labels to reduce clutter

B. Use only color changes to show errors

C. Hide error messages visually but keep them in the DOM

D. Add aria-live="polite" to error message container

Solution

1. Step 1: Understand accessibility for errors

   Screen readers need to be notified of error changes dynamically.

2. Step 2: Use aria-live attribute

   Adding aria-live="polite" tells assistive tech to announce updates politely.

3. Final Answer:

   Add aria-live="polite" to error message container -> Option D

4. Quick Check:

   aria-live improves screen reader announcements [OK]

Hint: Use aria-live for dynamic error announcements [OK]

Common Mistakes:

• Relying only on color changes (not accessible)
• Hiding errors visually removes info from users
• Removing labels harms form clarity