Component bindings in Svelte

Introduction

Component bindings let you connect data between a parent and child component so they stay in sync automatically.

You want a parent component to know when a child component's value changes.

You want to update a child component's value from the parent and keep it reactive.

You want to share form input values between components easily.

You want to keep UI elements like sliders or checkboxes synced across components.

Syntax

Svelte

<Child bind:property={parentVariable} />

The bind: directive connects a child component's property to a parent variable.

Changes in either the parent or child update the other automatically.

Examples

This binds the value property of the Input component to the name variable in the parent.

Svelte

<Input bind:value={name} />

The position property of Slider stays synced with sliderPos in the parent.

Svelte

<Slider bind:position={sliderPos} />

The checked state of the checkbox is bound to isChecked in the parent.

Svelte

<CustomCheckbox bind:checked={isChecked} />

Sample Program

This example shows a parent component with a message variable bound to the text property of the Child component. The child has an input box bound to text. Typing in the input updates message in the parent, and changes to message update the input.

Svelte

<script>
  import Child from './Child.svelte';
  let message = 'Hello';
</script>

<p>Parent message: {message}</p>
<Child bind:text={message} />

<!-- Child.svelte -->
<script>
  export let text;
</script>
<input bind:value={text} aria-label="Child input" />

Important Notes

Use bind: only on properties that the child component explicitly exports.

Bindings keep data in sync both ways without extra code.

Remember to add accessible labels like aria-label for inputs inside child components.

Summary

Component bindings connect parent and child data automatically.

Use bind:property to sync values both ways.

This makes UI updates simple and reactive without manual event handling.

Practice

1. What does bind:value do in a Svelte component?

easy

A. It creates a two-way connection between a variable and an input's value.

B. It only sets the initial value of the input without updating.

C. It listens for clicks on the input element.

D. It disables the input field.

Solution

1. Step 1: Understand bind:value usage

   In Svelte, bind:value links a variable to an input's value so changes update both ways.

2. Step 2: Compare options with behavior

   Only It creates a two-way connection between a variable and an input's value. describes this two-way syncing correctly; others describe unrelated behaviors.

3. Final Answer:

   It creates a two-way connection between a variable and an input's value. -> Option A

4. Quick Check:

   Two-way binding = It creates a two-way connection between a variable and an input's value. [OK]

Hint: Remember: bind:value syncs input and variable both ways [OK]

Common Mistakes:

• Thinking bind:value only sets initial value
• Confusing bind:value with event listeners
• Assuming bind:value disables input

2. Which of the following is the correct syntax to bind a parent's variable count to a child's prop value in Svelte?

easy

A.

B. count

C.

D.

Solution

1. Step 1: Recall Svelte binding syntax

   To bind a parent's variable to a child's prop, use bind:prop={variable} syntax.

2. Step 2: Evaluate options

   <Child bind:value={count} /> matches correct syntax. <Child value={count} /> passes value one-way. Options B and D have invalid syntax.

3. Final Answer:

   <Child bind:value={count} /> -> Option D

4. Quick Check:

   Correct bind syntax = <Child bind:value={count} /> [OK]

Hint: Use bind:prop={variable} to sync parent and child [OK]

Common Mistakes:

• Using curly braces without bind:
• Trying to bind without ={}
• Passing value as child content instead of prop

3. Given this Svelte code:

<script>
  let name = 'Alice';
</script>

<Child bind:name />

<!-- Child.svelte -->
<script>
  export let name;
</script>

<input bind:value={name} />

What happens when the user types 'Bob' in the input?

medium

A. An error occurs because bind:name is invalid.

B. Only the child's name updates; parent stays 'Alice'.

C. The parent's name variable updates to 'Bob' automatically.

D. The input value resets to 'Alice' immediately.

Solution

1. Step 1: Understand two-way binding between parent and child

   The parent binds name to child's name prop with bind:name, syncing both variables.

2. Step 2: Input binding updates child's name and propagates to parent

   Typing in input updates child's name, which updates parent's name automatically.

3. Final Answer:

   The parent's name variable updates to 'Bob' automatically. -> Option C

4. Quick Check:

   Two-way bind updates parent and child = The parent's name variable updates to 'Bob' automatically. [OK]

Hint: bind:prop syncs parent and child variables both ways [OK]

Common Mistakes:

• Thinking only child updates
• Expecting input to reset on typing
• Believing bind:name is invalid syntax

4. Identify the error in this Svelte code snippet:

<script>
  let age = 30;
</script>

<Child bind:age></Child>

<!-- Child.svelte -->
<script>
  export let age;
</script>

<input value={age} />

medium

A. The child input should use bind:value={age} for two-way binding.

B. The parent variable age must be a string, not a number.

C. The child component must not export age to bind it.

D. The parent should use bind:age={age} instead.

Solution

1. Step 1: Check child input binding

   The child input uses value={age}, which is one-way binding only.

2. Step 2: Fix to two-way binding for sync

   To sync changes back to parent, child input must use bind:value={age}.

3. Final Answer:

   The child input should use bind:value={age} for two-way binding. -> Option A

4. Quick Check:

   Two-way input binding needed = The child input should use bind:value={age} for two-way binding. [OK]

Hint: Use bind:value on inputs for two-way sync, not value= alone [OK]

Common Mistakes:

• Using value= instead of bind:value= on inputs
• Thinking export let disables binding
• Misusing bind:age syntax on parent

5. You want to create a custom slider component that updates a parent's volume variable as the slider moves. Which approach correctly uses Svelte component bindings to achieve this?

hard

A. In parent: <Slider value={volume} />In Slider.svelte: export let value; and <input type='range' value={value} />

B. In parent: <Slider bind:value={volume} />In Slider.svelte: export let value; and <input type='range' bind:value={value} />

C. In parent: <Slider bind:volume={volume} />In Slider.svelte: export let volume; and <input type='range' bind:value={volume} />

D. In parent: <Slider bind:value />In Slider.svelte: let value; and <input type='range' bind:value={value} />

Solution

1. Step 1: Understand binding prop names

   The parent binds its volume variable to child's value prop using bind:value={volume}.

2. Step 2: Child exports value and binds input's value to it

   The child exports value and uses bind:value={value} on the input for two-way sync.

3. Step 3: Check other options for errors

   In parent: <Slider value={volume} />
   In Slider.svelte: export let value; and <input type='range' value={value} /> uses one-way binding only. In parent: <Slider bind:volume={volume} />
   In Slider.svelte: export let volume; and <input type='range' bind:value={volume} /> mismatches prop names. In parent: <Slider bind:value />
   In Slider.svelte: let value; and <input type='range' bind:value={value} /> misses binding variable in parent.

4. Final Answer:

   Parent binds volume to child's value; child exports value and binds input value. -> Option B

5. Quick Check:

   Matching bind names and two-way input binding = In parent: <Slider bind:value={volume} />
   In Slider.svelte: export let value; and <input type='range' bind:value={value} /> [OK]

Hint: Bind parent's variable to child's exported prop with same name [OK]

Common Mistakes:

• Mismatching prop names between parent and child
• Using one-way binding instead of bind:value
• Not exporting prop in child component