Checkbox and radio bindings in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you bind a single checkbox to a boolean variable in Svelte?

Use the bind:checked directive on the checkbox input to link it to a boolean variable. For example: <input type="checkbox" bind:checked={isChecked} />.

intermediate

What happens when you bind multiple checkboxes to an array in Svelte?

Each checkbox's value is added to or removed from the array automatically as the user checks or unchecks them. Use bind:group on checkboxes to bind them to an array.

beginner

How do you bind a group of radio buttons to a single variable in Svelte?

Use bind:group on each radio input with the same variable. The variable will hold the value of the selected radio button.

beginner

What type of variable should you use to bind to a group of radio buttons?

Use a string or number variable that matches the value attribute of the radio buttons. This variable stores the selected option's value.

intermediate

Can you bind a checkbox to a variable other than boolean or array in Svelte?

No. A single checkbox binds to a boolean, and multiple checkboxes bind to an array. Other types are not supported for checkbox bindings.

Which directive binds a checkbox's checked state to a variable in Svelte?

A bind:checked

B bind:value

C bind:group

D bind:selected

The bind:checked directive links a checkbox's checked state to a boolean variable.

How do you bind multiple checkboxes to a single array variable in Svelte?

A Use bind:value on each checkbox

B Use bind:checked on each checkbox

C Use bind:group on each checkbox

D Use bind:selected on each checkbox

The bind:group directive binds multiple checkboxes to an array variable, managing the array automatically.

What value does a variable bound to a radio group hold?

A Array of selected values

B Value of the selected radio button

C Boolean true or false

D Number of selected radios

A variable bound to a radio group holds the value of the selected radio button.

Which input type uses bind:checked for binding in Svelte?

A Radio buttons

B Text inputs

C Select dropdowns

D Checkboxes

bind:checked is used for checkboxes to bind their checked state.

If you want to bind a group of radio buttons, which directive do you use?

A bind:group

B bind:value

C bind:checked

D bind:selected

Use bind:group to bind a group of radio buttons to a single variable.

Explain how to bind a single checkbox and multiple checkboxes in Svelte. Include the variable types used.

Think about how one checkbox differs from many checkboxes in binding.

Describe how to bind a group of radio buttons in Svelte and what the bound variable represents.

Consider how radio buttons let you pick only one option.

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