Form validation in Svelte - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to bind the input value to the variable.

Svelte

<input type="text" bind:value=[1] />

Drag options to blanks, or click blank then click option'

Apassword

Busername

Cemail

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable that is not declared.

Forgetting to use bind:value.

Binding the input's value to username allows Svelte to track the input's content.

2fill in blank

medium

Complete the code to check if the password length is at least 8 characters.

Svelte

{#if [1].length >= 8}
  <p>Password is strong enough.</p>
{/if}

Drag options to blanks, or click blank then click option'

Ausername

Bage

Cemail

Dpassword

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the wrong variable.

Using length without accessing the variable.

We check the length of the password variable to validate its strength.

3fill in blank

hard

Fix the error in the form submission handler to prevent default behavior.

Svelte

<form on:submit=[1]>
  <!-- form fields -->
</form>

Drag options to blanks, or click blank then click option'

AhandleSubmit

BhandleSubmit(event)

ChandleSubmit.prevent

DhandleSubmit|preventDefault

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the handler with parentheses in the template.

Not preventing the default form submission.

Using on:submit|preventDefault stops the form from reloading the page on submit.

4fill in blank

hard

Fill both blanks to create a reactive statement that updates isValid when email changes.

Svelte

<script>
  let email = '';
  let isValid = false;

  $: isValid = [1].includes('@') && [2].includes('.');
</script>

Drag options to blanks, or click blank then click option'

Aemail

Busername

Cpassword

Dage

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables in the reactive statement.

Not using the reactive statement syntax.

The reactive statement updates isValid based on the email variable's content.

5fill in blank

hard

Fill all three blanks to create a form with validation that disables the submit button if the form is invalid.

Svelte

<script>
  let email = '';
  let password = '';
  $: isFormValid = [1].includes('@') && [2].length >= 8;
</script>

<form>
  <input type="email" bind:value=[3] aria-label="Email address" required />
  <input type="password" bind:value={password} aria-label="Password" required />
  <button type="submit" disabled={!isFormValid}>Submit</button>
</form>

Drag options to blanks, or click blank then click option'

Aemail

Bpassword

Dusername

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong variables in validation.

Not binding the input values correctly.

The form checks if email contains '@' and password is at least 8 characters. The submit button disables if invalid.

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