Checkbox and radio bindings in Svelte - Step-by-Step Execution

Concept Flow - Checkbox and radio bindings

User clicks checkbox/radio

↓

Svelte updates bound variable

↓

Component re-renders with new state

↓

UI reflects updated checked state

When a user clicks a checkbox or radio button, Svelte updates the bound variable automatically, then re-renders the component to show the new checked state.

Execution Sample

Svelte

<script>
  let selected = '';
  let checked = false;
</script>
<input type="checkbox" bind:checked={checked}>
<input type="radio" bind:group={selected} value="A">

This code binds a checkbox to a boolean and a radio button to a string variable, updating variables on user interaction.

Execution Table

Step	User Action	Variable Before	Variable After	UI Checked State
1	Page loads	checked: false, selected: ''	checked: false, selected: ''	Checkbox unchecked, Radio none selected
2	User clicks checkbox	checked: false	checked: true	Checkbox checked
3	User clicks radio 'A'	selected: ''	selected: 'A'	Radio 'A' selected
4	User clicks checkbox again	checked: true	checked: false	Checkbox unchecked
5	User clicks radio 'A' again	selected: 'A'	selected: 'A'	Radio 'A' remains selected

💡 No more user actions; variables reflect last UI state.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
checked	false	true	true	false	false
selected	''	''	'A'	'A'	'A'

Key Moments - 3 Insights

Why does the checkbox variable 'checked' change when I click it?

Because the checkbox uses bind:checked, Svelte automatically updates the 'checked' variable when the user clicks, as shown in execution_table step 2.

Why does the radio variable 'selected' change only when I click a radio button?

The radio uses bind:group, so 'selected' updates only when a radio button in the group is clicked, as seen in execution_table step 3.

Why doesn't clicking the radio button again change the variable?

Clicking the already selected radio doesn't change the variable because it is already set to that value, shown in execution_table step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'checked' after step 4?

A undefined

B true

C false

D null

💡 Hint

Check the 'Variable After' column for step 4 in the execution_table.

At which step does the 'selected' variable first change from empty string to 'A'?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Variable After' column for 'selected' in execution_table rows.

If the user never clicks the checkbox, what will be the value of 'checked' after step 5?

A true

B false

C undefined

D null

💡 Hint

Refer to variable_tracker for 'checked' when no checkbox clicks occur.

Concept Snapshot

Checkbox and radio bindings in Svelte:
- Use bind:checked for checkboxes (boolean variable).
- Use bind:group for radio buttons (shared string variable).
- User clicks update variables automatically.
- UI updates reflect variable changes instantly.
- No manual event handling needed.

Full Transcript

This visual execution shows how Svelte binds checkboxes and radio buttons to variables. When the page loads, variables are at their initial values. Clicking the checkbox toggles the 'checked' boolean variable, and clicking a radio button sets the 'selected' string variable. The UI updates automatically to reflect these changes. Re-clicking a selected radio does not change the variable. This behavior is tracked step-by-step in the execution table and variable tracker, helping beginners see how user actions affect state and UI in Svelte.