Each blocks ({#each}) in Svelte - Performance & Optimization

Performance: Each blocks ({#each})

MEDIUM IMPACT

This affects how many DOM nodes are created and updated during rendering, impacting page load and interaction speed.

Rendering a list of items in Svelte

Svelte

<script>
  import { onMount } from 'svelte';
  let items = [];
  let visibleCount = 20;
  onMount(() => {
    items = Array(1000).fill(0).map((_, i) => i);
  });
</script>

<ul>
  {#each items.slice(0, visibleCount) as item}
    <li>{item}</li>
  {/each}
</ul>

Rendering only a small subset of items reduces DOM nodes and layout cost, improving load and interaction speed.

📈 Performance GainTriggers 20 reflows and paints instead of 1000, reducing blocking time by over 80%.

Rendering a list of items in Svelte

Svelte

<script>
  let items = Array(1000).fill(0).map((_, i) => i);
</script>

<ul>
  {#each items as item}
    <li>{item}</li>
  {/each}
</ul>

Rendering 1000 items at once creates many DOM nodes and triggers a large layout and paint cost, slowing initial load and interaction.

📉 Performance CostTriggers 1000 reflows and paints on initial render, blocking rendering for 100+ ms on typical devices.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Render all items at once	1000 nodes created	1000 reflows	High paint cost	[X] Bad
Render limited items with slice	20 nodes created	20 reflows	Low paint cost	[OK] Good

Rendering Pipeline

The {#each} block creates DOM nodes for each item. The browser must calculate styles, layout, and paint for all these nodes. Large lists increase the cost in Layout and Paint stages.

→DOM Construction

→Style Calculation

→Layout

→Paint

⚠️ BottleneckLayout and Paint stages due to many DOM nodes

Core Web Vital Affected

LCP, INP

This affects how many DOM nodes are created and updated during rendering, impacting page load and interaction speed.

Optimization Tips

1Avoid rendering very large lists all at once with {#each}.

2Use slicing or virtualization to limit rendered items.

3Monitor layout and paint costs in DevTools Performance panel.

Performance Quiz - 3 Questions

What happens if you render a very large list with {#each} without optimization?

A The page will load instantly with no performance impact.

B The page may load slowly due to many DOM nodes and layout calculations.

C The browser will automatically optimize and render only visible items.

D The list items will not appear on the page.

DevTools: Performance

How to check: Record a performance profile while loading the page with the {#each} block. Look at the Main thread activity and Frames section.

What to look for: Long tasks and many layout and paint events indicate heavy DOM work. Fewer and shorter tasks mean better performance.

Practice

1. What does the {#each} block do in Svelte?

easy

A. Creates a conditional statement to show or hide content

B. Repeats a block of HTML for each item in a list

C. Defines a reusable component

D. Declares a reactive variable

Solution

1. Step 1: Understand the purpose of {#each}

   The {#each} block is used to repeat content for every item in a list or array.

2. Step 2: Compare with other options

   Options A, C, and D describe other Svelte features, not list repetition.

3. Final Answer:

   Repeats a block of HTML for each item in a list -> Option B

4. Quick Check:

   {#each} = repeat list items [OK]

Hint: Remember: {#each} loops over lists [OK]

Common Mistakes:

• Confusing {#each} with {#if}
• Thinking it creates components
• Assuming it declares variables

2. Which of the following is the correct syntax to loop over an array items and display each item in Svelte?

easy

A. {#each items to item} <p>{item}</p> {/each}

B. {#each item in items} <p>{item}</p> {/each}

C. {#each items for item} <p>{item}</p> {/each}

D. {#each items as item} <p>{item}</p> {/each}

Solution

1. Step 1: Recall Svelte each block syntax

   The correct syntax is {#each array as item} to loop over an array.

2. Step 2: Check each option

   Only {#each items as item} <p>{item}</p> {/each} uses as correctly. Options A, B, and C use incorrect keywords.

3. Final Answer:

   {#each items as item} <p>{item}</p> {/each} -> Option D

4. Quick Check:

   Use as keyword in {#each} [OK]

Hint: Use 'as' keyword: {#each array as item} [OK]

Common Mistakes:

• Using 'in' instead of 'as'
• Using 'for' or 'to' keywords
• Missing the 'as' keyword

3. Given the code:

{`


  {#each fruits as fruit, i}
    
{i}: {fruit}
  {/each}
`}

What will be the rendered output inside the <ul>?

medium

A. 0: apple1: banana2: cherry

B. apple: 0banana: 1cherry: 2

C. 1: apple2: banana3: cherry

D. fruit: applefruit: bananafruit: cherry

Solution

1. Step 1: Understand the each block variables

   The loop uses fruit for the item and i for the index starting at 0.

2. Step 2: Match output format

   Each list item shows index, colon, then fruit name, so output is 0: apple, 1: banana, 2: cherry.

3. Final Answer:

   <li>0: apple</li><li>1: banana</li><li>2: cherry</li> -> Option A

4. Quick Check:

   Index starts at 0, item after colon [OK]

Hint: Index starts at 0 in {#each items as item, i} [OK]

Common Mistakes:

• Confusing index and item order
• Starting index at 1 instead of 0
• Using wrong variable names

4. What is wrong with this Svelte code snippet?

{`


  {#each numbers as number}
    
{num}
  {/each}
`}

medium

A. The variable inside the block should be 'number', not 'num'

B. The array 'numbers' is not defined

C. The each block syntax is incorrect

D. Missing closing {/each} tag

Solution

1. Step 1: Check variable names inside each block

   The loop defines number as the item, but inside the block it uses {num}, which is undefined.

2. Step 2: Verify other syntax parts

   The array is defined, the each block syntax and closing tag are correct.

3. Final Answer:

   The variable inside the block should be 'number', not 'num' -> Option A

4. Quick Check:

   Use same variable name inside each block [OK]

Hint: Use the exact loop variable name inside the block [OK]

Common Mistakes:

• Using a different variable name inside the block
• Forgetting to close the each block
• Misnaming the array variable

5. You want to render a list of users with unique IDs and update the list efficiently when users change. Which is the best way to use {#each} with keys in Svelte?

hard

A. {#each users as user (user.name)} <p>{user.name}</p> {/each}

B. {#each users as user} <p>{user.name}</p> {/each}

C. {#each users as user (user.id)} <p>{user.name}</p> {/each}

D. {#each users as user (index)} <p>{user.name}</p> {/each}

Solution

1. Step 1: Understand keys in each blocks

   Keys help Svelte track items uniquely for efficient updates. The key should be a unique, stable identifier like user.id.

2. Step 2: Evaluate options

   {#each users as user (user.id)} <p>{user.name}</p> {/each} uses user.id as key, which is best. {#each users as user} <p>{user.name}</p> {/each} has no key, less efficient. {#each users as user (user.name)} <p>{user.name}</p> {/each} uses user.name, which may not be unique. {#each users as user (index)} <p>{user.name}</p> {/each} uses index, which can cause issues when list changes.

3. Final Answer:

   {#each users as user (user.id)} <p>{user.name}</p> {/each} -> Option C

4. Quick Check:

   Use unique ID as key in each block [OK]

Hint: Use unique IDs as keys: {#each items as item (item.id)} [OK]

Common Mistakes:

• Not using keys at all
• Using non-unique keys like names
• Using index as key causing update bugs