Checkbox and radio bindings in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when selecting multiple checkboxes?

Consider this Svelte component with checkboxes bound to an array. What will be the value of selected after checking "Apple" and "Cherry"?

Svelte

<script>
  let selected = [];
</script>

<label><input type="checkbox" bind:group={selected} value="Apple"> Apple</label>
<label><input type="checkbox" bind:group={selected} value="Banana"> Banana</label>
<label><input type="checkbox" bind:group={selected} value="Cherry"> Cherry</label>

A ["Banana", "Cherry"]

B ["Apple"]

C []

D ["Apple", "Cherry"]

💡 Hint

Checkboxes bound with bind:group add or remove values from the array.

When you check "Apple" and "Cherry", their values are added to the selected array. So the array contains both strings.

❓ component_behavior

intermediate

What is the value of a radio button binding?

Given this Svelte radio button group bound to choice, what is the value of choice after selecting the "Blue" option?

Svelte

<script>
  let choice = "Red";
</script>

<label><input type="radio" bind:group={choice} value="Red"> Red</label>
<label><input type="radio" bind:group={choice} value="Blue"> Blue</label>
<label><input type="radio" bind:group={choice} value="Green"> Green</label>

A "Blue"

B undefined

C "Green"

D "Red"

💡 Hint

Radio buttons bound with bind:group set the variable to the selected value.

Selecting the "Blue" radio button updates choice to "Blue".

📝 Syntax

advanced

Which option correctly binds multiple checkboxes to an array?

Which of the following Svelte code snippets correctly binds multiple checkboxes to an array named fruits?

A <input type="checkbox" bind:group={fruits} value="Apple">

B <input type="checkbox" bind:group={fruits} checked>

C <input type="checkbox" bind:checked={fruits} value="Apple">

D <input type="checkbox" bind:value={fruits} value="Apple">

💡 Hint

Use bind:group to bind checkboxes to an array.

Only bind:group correctly binds multiple checkboxes to an array. The others either bind a single value or are invalid.

🔧 Debug

advanced

Why does this radio button binding not update the variable?

This Svelte code tries to bind a radio group to color, but selecting options does not update color. What is the problem?

Svelte

<script>
  let color = "red";
</script>

<label><input type="radio" bind:group={color} value="red"> Red</label>
<label><input type="radio" bind:group={color} value="green"> Green</label>
<label><input type="radio" bind:group={color} value="blue"> Blue</label>

<!-- The variable color never changes on selection -->

A The <code>bind:group</code> is correct but the <code>value</code> attributes are missing quotes.

B The <code>value</code> attributes are strings but <code>color</code> is a number.

C The <code>value</code> attributes are lowercase strings but <code>color</code> initial value is lowercase string, so it should work.

D The variable <code>color</code> is declared with lowercase but values use uppercase.

💡 Hint

Check if the binding syntax and value types match.

The code is correct and should update color. If it does not, the problem is likely outside this snippet or a misunderstanding.

❓ state_output

expert

What is the final state of the array after toggling checkboxes?

Given this Svelte component, what is the value of selected after the user checks "Banana", then "Apple", then unchecks "Banana"?

Svelte

<script>
  let selected = [];
</script>

<label><input type="checkbox" bind:group={selected} value="Apple"> Apple</label>
<label><input type="checkbox" bind:group={selected} value="Banana"> Banana</label>
<label><input type="checkbox" bind:group={selected} value="Cherry"> Cherry</label>

A ["Banana", "Apple"]

B ["Apple"]

C ["Banana"]

D []

💡 Hint

Checkboxes add or remove their value from the array when toggled.

Checking "Banana" adds it, then "Apple" adds it, then unchecking "Banana" removes it, leaving only "Apple".

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