Checkbox and radio bindings in Svelte

Introduction

Checkbox and radio bindings let you easily connect form inputs to your code. This means your app knows what the user selects without extra work.

When you want to let users pick one or more options in a form.

When you need to update your app instantly as users check or select choices.

When building surveys, settings pages, or filters with multiple options.

When you want simple code that automatically keeps track of user selections.

Syntax

Svelte

<input type="checkbox" bind:checked={variable} />
<input type="radio" bind:group={variable} value="option1" />

Use bind:checked for checkboxes to link a boolean variable.

Use bind:group for radio buttons to link a variable that holds the selected value.

Examples

This example shows a checkbox linked to isChecked. The text updates as you check or uncheck.

Svelte

<script>
  let isChecked = false;
</script>
<input type="checkbox" bind:checked={isChecked} />
<p>Checked? {isChecked ? 'Yes' : 'No'}</p>

Here, radio buttons share the same group variable selectedColor. Only one can be selected at a time.

Svelte

<script>
  let selectedColor = '';
</script>
<label><input type="radio" bind:group={selectedColor} value="red" /> Red</label>
<label><input type="radio" bind:group={selectedColor} value="blue" /> Blue</label>
<p>Selected color: {selectedColor}</p>

Checkboxes with bind:group can link to an array to track multiple selections.

Svelte

<script>
  let fruits = [];
</script>
<label><input type="checkbox" bind:group={fruits} value="apple" /> Apple</label>
<label><input type="checkbox" bind:group={fruits} value="banana" /> Banana</label>
<p>Fruits selected: {fruits.join(', ')}</p>

Sample Program

This component shows three examples: a single checkbox, radio buttons, and multiple checkboxes bound to variables. The displayed text updates as you interact.

Svelte

<script>
  let agree = false;
  let gender = '';
  let hobbies = [];
</script>
<main>
  <h2>Checkbox binding</h2>
  <label>
    <input type="checkbox" bind:checked={agree} /> I agree to terms
  </label>
  <p>Agreed? {agree ? 'Yes' : 'No'}</p>

  <h2>Radio binding</h2>
  <label><input type="radio" bind:group={gender} value="male" /> Male</label>
  <label><input type="radio" bind:group={gender} value="female" /> Female</label>
  <p>Selected gender: {gender}</p>

  <h2>Multiple checkboxes with group</h2>
  <label><input type="checkbox" bind:group={hobbies} value="reading" /> Reading</label>
  <label><input type="checkbox" bind:group={hobbies} value="sports" /> Sports</label>
  <label><input type="checkbox" bind:group={hobbies} value="music" /> Music</label>
  <p>Hobbies: {hobbies.length > 0 ? hobbies.join(', ') : 'None'}</p>
</main>

Important Notes

Checkbox bind:checked links to a boolean (true/false).

Radio buttons use bind:group to share one variable for the selected value.

Multiple checkboxes can share bind:group with an array to track all checked items.

Summary

Checkboxes use bind:checked for true/false values.

Radio buttons use bind:group to select one value from many.

Multiple checkboxes can bind to an array with bind:group to track all selections.

Practice

1. In Svelte, which binding is used to track whether a single checkbox is checked or not?

easy

A. bind:group

B. bind:value

C. bind:checked

D. bind:selected

Solution

1. Step 1: Understand checkbox binding

   Checkboxes in Svelte use bind:checked to track true/false state.

2. Step 2: Differentiate from other bindings

   bind:value is for input values, bind:group is for radio or multiple checkboxes, bind:selected is invalid.

3. Final Answer:

   <code>bind:checked</code> -> Option C

4. Quick Check:

   Checkbox true/false = bind:checked [OK]

Hint: Checkbox true/false uses bind:checked [OK]

Common Mistakes:

• Using bind:value for checkbox state
• Confusing bind:group with bind:checked
• Trying bind:selected which doesn't exist

2. Which of the following is the correct syntax to bind a group of radio buttons to a variable color in Svelte?

easy

A. <input type='radio' bind:checked={color} value='red' />

B. <input type='radio' bind:selected={color} value='red' />

C. <input type='radio' bind:value={color} value='red' />

D. <input type='radio' bind:group={color} value='red' />

Solution

1. Step 1: Identify radio button binding

   Radio buttons use bind:group to bind multiple inputs to one variable.

2. Step 2: Check syntax correctness

   Only bind:group={color} correctly binds the group to variable color.

3. Final Answer:

   <input type='radio' bind:group={color} value='red' /> -> Option D

4. Quick Check:

   Radio group binding = bind:group [OK]

Hint: Radio buttons bind group with bind:group [OK]

Common Mistakes:

• Using bind:checked for radio groups
• Using bind:value instead of bind:group
• Using bind:selected which is invalid

3. Given this Svelte code:

<script>
  let selectedColors = [];
</script>
<input type='checkbox' bind:group={selectedColors} value='red' /> Red
<input type='checkbox' bind:group={selectedColors} value='blue' /> Blue
<input type='checkbox' bind:group={selectedColors} value='green' /> Green

<p>Selected: {JSON.stringify(selectedColors)}</p>

If the user checks 'red' and 'green', what will be displayed inside the paragraph?

medium

A. Selected: ["red", "green"]

B. Selected: ["red"]

C. Selected: [true, true]

D. Selected: red, green

Solution

1. Step 1: Understand bind:group with checkboxes

   Binding multiple checkboxes with bind:group stores selected values in an array.

2. Step 2: Check which values are selected

   User selects 'red' and 'green', so array contains ["red", "green"].

3. Final Answer:

   Selected: ["red", "green"] -> Option A

4. Quick Check:

   Checkbox group array = ["red", "green"] [OK]

Hint: Multiple checkboxes bind:group stores array of values [OK]

Common Mistakes:

• Expecting boolean array instead of values
• Confusing bind:checked with bind:group
• Outputting string instead of JSON array

4. Consider this Svelte snippet:

<script>
  let fruit = 'apple';
</script>
<input type='radio' bind:checked={fruit} value='apple' /> Apple
<input type='radio' bind:group={fruit} value='banana' /> Banana

What is the main issue with this code?

medium

A. Using bind:group with a string variable is invalid

B. Mixing bind:checked and bind:group on radio buttons causes inconsistent binding

C. Missing bind:value on radio inputs

D. Radio buttons must use bind:checked, not bind:group

Solution

1. Step 1: Identify bindings on radio buttons

   One radio uses bind:checked, the other uses bind:group on the same variable.

2. Step 2: Understand correct radio binding

   Radio buttons must all use bind:group to bind to one variable consistently.

3. Final Answer:

   Mixing bind:checked and bind:group on radio buttons causes inconsistent binding -> Option B

4. Quick Check:

   Radio group must use bind:group consistently [OK]

Hint: Use bind:group consistently for all radios in a group [OK]

Common Mistakes:

• Using bind:checked on radios instead of bind:group
• Mixing different bindings on radios
• Assuming bind:value binds radio group

5. You want to create a Svelte form where users can select multiple toppings from checkboxes and also pick a single pizza size with radio buttons. Which of the following code snippets correctly binds the toppings to an array selectedToppings and the size to a string pizzaSize?

hard

A. <script>
let selectedToppings = [];
let pizzaSize = '';
</script>
<input type='checkbox' bind:group={selectedToppings} value='pepperoni' /> Pepperoni
<input type='checkbox' bind:group={selectedToppings} value='mushrooms' /> Mushrooms
<input type='radio' bind:group={pizzaSize} value='small' /> Small
<input type='radio' bind:group={pizzaSize} value='large' /> Large

B. <script>
let selectedToppings = '';
let pizzaSize = [];
</script>
<input type='checkbox' bind:checked={selectedToppings} value='pepperoni' /> Pepperoni
<input type='checkbox' bind:checked={selectedToppings} value='mushrooms' /> Mushrooms
<input type='radio' bind:checked={pizzaSize} value='small' /> Small
<input type='radio' bind:checked={pizzaSize} value='large' /> Large

C. <script>
let selectedToppings = [];
let pizzaSize = '';
</script>
<input type='checkbox' bind:checked={selectedToppings} value='pepperoni' /> Pepperoni
<input type='checkbox' bind:checked={selectedToppings} value='mushrooms' /> Mushrooms
<input type='radio' bind:value={pizzaSize} value='small' /> Small
<input type='radio' bind:value={pizzaSize} value='large' /> Large

D. <script>
let selectedToppings = [];
let pizzaSize = '';
</script>
<input type='checkbox' bind:group={selectedToppings} value='pepperoni' /> Pepperoni
<input type='checkbox' bind:group={selectedToppings} value='mushrooms' /> Mushrooms
<input type='radio' bind:checked={pizzaSize} value='small' /> Small
<input type='radio' bind:checked={pizzaSize} value='large' /> Large

Solution

1. Step 1: Bind multiple checkboxes to an array

   Use bind:group on checkboxes to bind selected values to an array variable.

2. Step 2: Bind radio buttons to a string variable

   Use bind:group on radio buttons to bind the selected value to a string variable.

3. Step 3: Verify correct syntax in options

   <script>
     let selectedToppings = [];
     let pizzaSize = '';
   </script>
   <input type='checkbox' bind:group={selectedToppings} value='pepperoni' /> Pepperoni
   <input type='checkbox' bind:group={selectedToppings} value='mushrooms' /> Mushrooms
   <input type='radio' bind:group={pizzaSize} value='small' /> Small
   <input type='radio' bind:group={pizzaSize} value='large' /> Large

   correctly uses bind:group for both checkboxes and radios with proper variables.

4. Final Answer:

   Option A code snippet -> Option A

5. Quick Check:

   Checkbox array + radio string = bind:group [OK]

Hint: Use bind:group for checkbox arrays and radio groups [OK]

Common Mistakes:

• Using bind:checked for multiple checkboxes array
• Using bind:checked for radio groups
• Mixing bind:value and bind:checked incorrectly