svelte:window for window events - Mini Project: Build & Apply

Using <code>svelte:window</code> to Track Window Resize

📖 Scenario: You are building a simple Svelte app that shows the current width of the browser window. This helps users see how wide their screen is in real time.

🎯 Goal: Create a Svelte component that listens to window resize events using svelte:window and updates the displayed window width dynamically.

📋 What You'll Learn

Create a variable width to store the window width

Use svelte:window with the on:resize event

Update width when the window resizes

Display the current width in the component

💡 Why This Matters

🌍 Real World

Tracking window size is common in responsive web design to adjust layouts or content dynamically.

💼 Career

Understanding how to handle window events in Svelte is useful for building interactive, accessible web apps that respond to user environment changes.

1

Set up the width variable

Create a variable called width and set it to window.innerWidth to get the initial window width.

Svelte

let width = window.innerWidth
// Your code here

Hint

Use let width = window.innerWidth to get the current width of the browser window.

2

Add svelte:window with on:resize event

Add a <svelte:window on:resize={handleResize}></svelte:window> tag to listen for window resize events. Define a function called handleResize that updates width to window.innerWidth.

Svelte

let width = window.innerWidth

function handleResize() {
  // Your code here
}

Hint

Define handleResize to update width. Use <svelte:window on:resize={handleResize}> to listen for resize.

3

Display the current window width

Add a <p> tag that shows the text Window width: {width}px so users can see the current width.

Svelte

let width = window.innerWidth

function handleResize() {
  width = window.innerWidth
}

Hint

Use <p>Window width: {width}px</p> to show the width.

4

Make the component accessible and responsive

Add an aria-live="polite" attribute to the <p> tag so screen readers announce width changes politely.

Svelte

let width = window.innerWidth

function handleResize() {
  width = window.innerWidth
}



Window width: {width}px  

Hint

Add aria-live="polite" to the paragraph to improve accessibility.

Practice

1. What is the main purpose of using svelte:window in a Svelte component?

easy

A. To style the window element with CSS

B. To create a new browser window

C. To listen to events on the browser window like resize or scroll

D. To manage component state internally

Solution

1. Step 1: Understand svelte:window usage

   svelte:window is a special element in Svelte used to listen to events on the browser window object.

2. Step 2: Identify the correct purpose

   It does not create windows or style elements but listens to events like resize or scroll on the window.

3. Final Answer:

   To listen to events on the browser window like resize or scroll -> Option C

4. Quick Check:

   svelte:window listens to window events [OK]

Hint: Remember: svelte:window listens to window events only [OK]

Common Mistakes:

• Thinking it creates or styles windows
• Confusing with component state management
• Using it to listen to DOM element events

2. Which of the following is the correct syntax to listen for the window's resize event using svelte:window?

easy

A. <div on:resize={handleResize}></div>

B. <svelte:window on:resize={handleResize} />

C. <window onresize={handleResize}></window>

D. <svelte:window bind:resize={handleResize} />

Solution

1. Step 1: Recall correct event binding syntax in Svelte

   In Svelte, event listeners use on:eventName={handler} syntax.

2. Step 2: Apply to svelte:window

   The correct way to listen to window resize is <svelte:window on:resize={handleResize} />.

3. Final Answer:

   <svelte:window on:resize={handleResize} /> -> Option B

4. Quick Check:

   Use on:event on svelte:window [OK]

Hint: Use on:event syntax on svelte:window [OK]

Common Mistakes:

• Using HTML window tag which doesn't exist
• Trying to bind event with bind: instead of on:
• Attaching events to div instead of svelte:window

3. Given this Svelte code snippet, what will be the value of width after resizing the browser window?

  <script>
    let width = 0;
    function updateWidth(event) {
      width = event.currentTarget.innerWidth;
    }
  </script>

  <svelte:window on:resize={updateWidth} />

  <p>Window width: {width}</p>

medium

A. It updates to the current window width on every resize

B. It stays 0 because width is never updated

C. It throws an error because event.currentTarget is undefined

D. It updates only once when the component loads

Solution

1. Step 1: Understand the event handler logic

   The updateWidth function sets width to event.currentTarget.innerWidth, which is the window's width.

2. Step 2: Recognize svelte:window event behavior

   On every resize event, updateWidth runs, updating width reactively.

3. Final Answer:

   It updates to the current window width on every resize -> Option A

4. Quick Check:

   Resize triggers updateWidth, updating width [OK]

Hint: Resize triggers handler updating width reactively [OK]

Common Mistakes:

• Assuming width never changes without explicit initialization
• Confusing event.currentTarget with event.target
• Thinking event.currentTarget is undefined in this context

4. Identify the error in this Svelte code that tries to listen for the window scroll event:

  <script>
    let scrollY = 0;
    function onScroll() {
      scrollY = window.scrollY;
    }
  </script>

  <svelte:window on:scroll={onScroll}></svelte:window>

  <p>Scroll position: {scrollY}</p>

medium

A. Not binding scrollY with bind:scrollY

B. Missing event parameter in onScroll function

C. Accessing window.scrollY inside the handler is invalid

D. Using <svelte:window> with a closing tag instead of self-closing

Solution

1. Step 1: Check svelte:window tag usage

   svelte:window is a special element that must be self-closing when used to listen to events.

2. Step 2: Identify the tag error

   The code uses an opening and closing tag <svelte:window></svelte:window>, which is invalid syntax for this element.

3. Final Answer:

   Using <svelte:window> with a closing tag instead of self-closing -> Option D

4. Quick Check:

   svelte:window must be self-closing [OK]

Hint: Always self-close svelte:window when listening to events [OK]

Common Mistakes:

• Trying to use opening and closing tags for svelte:window
• Thinking bind: is needed for event values
• Assuming window.scrollY is inaccessible in handler

5. You want to track both the window's width and height reactively using svelte:window. Which code snippet correctly updates both values on resize events?

hard

A. <script>
let width = 0, height = 0;
function updateSize() {
width = window.innerWidth;
height = window.innerHeight;
}
</script>
<svelte:window on:resize={updateSize} />

B. <script>
let width = 0, height = 0;
function updateSize(event) {
width = event.currentTarget.innerWidth;
height = event.currentTarget.innerHeight;
}
</script>
<svelte:window on:resize={updateSize} />

C. <script>
let width = 0, height = 0;
<svelte:window bind:innerWidth={width} bind:innerHeight={height} />
</script>

D. <script>
let width = 0, height = 0;
function updateSize(event) {
width = event.target.innerWidth;
height = event.target.innerHeight;
}
</script>
<svelte:window on:resize={updateSize} />

Solution

1. Step 1: Understand event object properties

   In window resize events, event.currentTarget and event.target may not reliably provide innerWidth and innerHeight. Using window.innerWidth and window.innerHeight is safer.

2. Step 2: Check each option

   <script> let width = 0, height = 0; function updateSize(event) { width = event.currentTarget.innerWidth; height = event.currentTarget.innerHeight; } </script> <svelte:window on:resize={updateSize} /> uses event.currentTarget which may not always be the window. <script> let width = 0, height = 0; function updateSize() { width = window.innerWidth; height = window.innerHeight; } </script> <svelte:window on:resize={updateSize} /> uses window.innerWidth and window.innerHeight directly, which is correct. <script> let width = 0, height = 0; <svelte:window bind:innerWidth={width} bind:innerHeight={height} /> </script> tries to bind non-bindable properties. <script> let width = 0, height = 0; function updateSize(event) { width = event.target.innerWidth; height = event.target.innerHeight; } </script> <svelte:window on:resize={updateSize} /> uses event.target which is unreliable.

3. Final Answer:

   Use window.innerWidth and window.innerHeight inside the handler -> Option A

4. Quick Check:

   Access window size via window.innerWidth in handler [OK]

Hint: Use window.innerWidth inside handler, not event properties [OK]

Common Mistakes:

• Using event.currentTarget or event.target for window size
• Trying to bind window properties directly
• Assuming event object always has innerWidth/innerHeight