Group bindings in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Svelte group binding example?

Consider this Svelte component using group binding with checkboxes:

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

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

<p>Selected: {selected.join(', ')}</p>

If the user checks only the "banana" and "cherry" boxes, what will be displayed inside the <p> tag?

A Selected: apple

B Selected: apple, banana, cherry

C Selected: banana, cherry

D Selected:

💡 Hint

Group binding collects all checked values into the array.

Only the checked checkboxes add their values to the selected array. Since "banana" and "cherry" are checked, the output lists those two.

❓ state_output

intermediate

What is the value of the bound variable after toggling checkboxes?

Given this Svelte snippet:

<script>
  let fruits = ['apple'];
</script>

<input type="checkbox" bind:group={fruits} value="apple" /> Apple
<input type="checkbox" bind:group={fruits} value="banana" /> Banana
<input type="checkbox" bind:group={fruits} value="cherry" /> Cherry

Initially, only "apple" is checked. The user then checks "banana" and unchecks "apple". What is the value of fruits after these changes?

A ['banana']

B ['banana', 'cherry']

C ['apple', 'banana']

D ['apple']

💡 Hint

Group binding updates the array to include only checked values.

Unchecking "apple" removes it from the array. Checking "banana" adds it. "cherry" remains unchecked, so it is not included.

📝 Syntax

advanced

Which option correctly uses group binding with radio buttons in Svelte?

Choose the correct Svelte code snippet that binds a group of radio buttons to a variable color:

A

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

B

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

C

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

D

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

💡 Hint

Radio buttons use bind:group to bind a single variable to the selected value.

Option A correctly binds the color variable to the selected radio button using bind:group. Option A uses invalid syntax. Option A binds to colors which is undefined. Option A uses checkboxes instead of radios.

🔧 Debug

advanced

Why does this Svelte group binding code cause an error?

Examine this Svelte code snippet:

<script>
  let selected = '';
</script>

<input type="checkbox" bind:group={selected} value="one" /> One
<input type="checkbox" bind:group={selected} value="two" /> Two

When running, it throws an error. What is the cause?

A The bind:group directive is missing a colon.

B Checkbox inputs cannot use bind:group, only radio inputs can.

C The value attribute must be a number, not a string.

D The variable 'selected' must be an array, not a string, for checkbox group binding.

💡 Hint

Check the type of the variable used with bind:group for checkboxes.

Checkbox group binding requires the bound variable to be an array to hold multiple selected values. Using a string causes a runtime error.

🧠 Conceptual

expert

How does Svelte's group binding handle dynamic changes to the bound array?

In Svelte, if you bind a group of checkboxes to an array variable using bind:group, and then programmatically modify the array (e.g., add or remove values), what happens to the checkboxes?

A The checkboxes do not update; only user interaction changes their state.

B The checkboxes update automatically to reflect the current array contents, checking or unchecking accordingly.

C Modifying the array causes a runtime error because the binding is one-way.

D Only the first checkbox updates; others remain unchanged.

💡 Hint

Think about two-way binding and reactivity in Svelte.

Svelte's bind:group creates a two-way binding. Changing the array programmatically updates the checkboxes to match the array's contents automatically.

Practice

1. In Svelte, what does a group binding do when used with multiple checkboxes?

easy

A. It connects all checkboxes to a single array variable that holds selected values.

B. It links each checkbox to a separate variable automatically.

C. It disables all checkboxes except one.

D. It creates a radio button group instead of checkboxes.

Solution

1. Step 1: Understand group binding with checkboxes

   Group binding in Svelte with checkboxes uses an array variable to store all selected checkbox values.

2. Step 2: Recognize the effect on state

   When a checkbox is checked or unchecked, the array updates automatically to reflect the current selections.

3. Final Answer:

   It connects all checkboxes to a single array variable that holds selected values. -> Option A

4. Quick Check:

   Checkbox group binding = array variable [OK]

Hint: Checkbox groups bind to arrays holding all selected values [OK]

Common Mistakes:

• Thinking each checkbox needs its own variable
• Confusing checkbox groups with radio groups
• Assuming group binding disables inputs

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

easy

A.

B.

C.

D.

Solution

1. Step 1: Recall Svelte group binding syntax

   For radio buttons, Svelte uses bind:group to connect all radios to one variable.

2. Step 2: Match syntax with options

   uses bind:group={selected}, which is the correct syntax for radio groups.

3. Final Answer:

   <input type="radio" bind:group={selected} value="A"> -> Option A

4. Quick Check:

   Radio group binding syntax = bind:group [OK]

Hint: Use bind:group for radio button groups in Svelte [OK]

Common Mistakes:

• Using bind:value instead of bind:group
• Using bind:checked which is for single checkbox
• Incorrect attribute order or spelling

3. Consider this Svelte code snippet:

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

<label><input type="checkbox" bind:group={selectedFruits} value="apple"> Apple</label>
<label><input type="checkbox" bind:group={selectedFruits} value="banana"> Banana</label>
<label><input type="checkbox" bind:group={selectedFruits} value="cherry"> Cherry</label>

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

If the user checks Apple and Cherry, what will be displayed inside the <p> tag?

medium

A. Selected: {apple, cherry}

B. Selected: ["banana"]

C. Selected: apple,cherry

D. Selected: ["apple", "cherry"]

Solution

1. Step 1: Understand checkbox group binding behavior

   When checkboxes are bound to an array, each checked box's value is added to the array.

2. Step 2: Identify selected checkboxes and output format

   User checked Apple and Cherry, so array is ["apple", "cherry"]. JSON.stringify shows this as a JSON array string.

3. Final Answer:

   Selected: ["apple", "cherry"] -> Option D

4. Quick Check:

   Checkbox group array output = JSON array string [OK]

Hint: Checkbox group values appear as array strings in output [OK]

Common Mistakes:

• Expecting a comma-separated string instead of array
• Confusing selected values with unchecked ones
• Misreading JSON.stringify output format

4. This Svelte code tries to bind a group of checkboxes but does not work as expected:

<script>
  let selected = "";
</script>

<input type="checkbox" bind:group={selected} value="x">
<input type="checkbox" bind:group={selected} value="y">

What is the main problem here?

medium

A. The value attribute is missing from the inputs.

B. The variable 'selected' should be an array, not a string, for checkbox group binding.

C. Checkbox inputs cannot use bind:group in Svelte.

D. bind:group only works with radio buttons, not checkboxes.

Solution

1. Step 1: Check variable type for checkbox group binding

   Checkbox group binding requires the bound variable to be an array to hold multiple selected values.

2. Step 2: Identify the error in variable initialization

   The code uses a string variable instead of an array, causing binding to fail or behave incorrectly.

3. Final Answer:

   The variable 'selected' should be an array, not a string, for checkbox group binding. -> Option B

4. Quick Check:

   Checkbox group variable must be array [OK]

Hint: Checkbox group variables must be arrays, not strings [OK]

Common Mistakes:

• Using string instead of array for checkbox group
• Thinking bind:group is only for radios
• Forgetting to set value attribute on inputs

5. You want to create a Svelte form where users select their favorite colors from checkboxes and also pick one preferred shape from radio buttons. Which is the best way to bind these inputs to variables colors and shape respectively?

hard

A. let colors = []; with bind:group={colors} on radios, and let shape = ''; with bind:group={shape} on checkboxes.

B. let colors = ''; with bind:value={colors} on checkboxes, and let shape = []; with bind:group={shape} on radios.

C. let colors = []; with bind:group={colors} on checkboxes, and let shape = ''; with bind:group={shape} on radios.

D. let colors = ''; with bind:group={colors} on checkboxes, and let shape = ''; with bind:value={shape} on radios.

Solution

1. Step 1: Assign correct variable types for inputs

   Checkbox groups bind to arrays to hold multiple selections; radio groups bind to single values (strings).

2. Step 2: Match bindings to input types

   Checkboxes use bind:group with an array variable; radios use bind:group with a string variable.

3. Final Answer:

   let colors = [] with bind:group on checkboxes, let shape = '' with bind:group on radios -> Option C

4. Quick Check:

   Checkboxes = array, radios = string with bind:group [OK]

Hint: Checkbox groups use arrays; radio groups use single values [OK]

Common Mistakes:

• Swapping variable types between checkboxes and radios
• Using bind:value instead of bind:group for groups
• Initializing variables with wrong types