Middleware patterns with hooks in Svelte

Introduction

Middleware patterns with hooks help you run code before or after certain actions in your app. This lets you add features like logging, validation, or data fetching in a clean way.

You want to check user permissions before showing a page.

You need to log user actions like clicks or form submissions.

You want to fetch data automatically when a component loads.

You want to modify or validate data before saving it.

You want to run cleanup code when a component is removed.

Syntax

Svelte

import { beforeUpdate, afterUpdate, onMount, onDestroy } from 'svelte';

// Example middleware hook usage
onMount(() => {
  // Code to run when component appears
});

beforeUpdate(() => {
  // Code to run before DOM updates
});

afterUpdate(() => {
  // Code to run after DOM updates
});

onDestroy(() => {
  // Cleanup code when component is removed
});

Hooks like onMount and onDestroy let you run code at specific times in a component's life.

You can think of these hooks as middleware that runs around your component's main work.

Examples

This runs code once when the component appears on the page.

Svelte

import { onMount } from 'svelte';

onMount(() => {
  console.log('Component loaded');
});

This runs code right before the component updates the page.

Svelte

import { beforeUpdate } from 'svelte';

beforeUpdate(() => {
  console.log('About to update DOM');
});

This runs cleanup code when the component is removed from the page.

Svelte

import { onDestroy } from 'svelte';

onDestroy(() => {
  console.log('Component is gone, cleaning up');
});

Sample Program

This Svelte component uses middleware hooks to log messages at different times: when it loads, before and after updates, and when it is removed. Clicking the button changes the count and triggers the update hooks.

Svelte

<script>
  import { onMount, beforeUpdate, afterUpdate, onDestroy } from 'svelte';

  let count = 0;

  onMount(() => {
    console.log('Component mounted');
  });

  beforeUpdate(() => {
    console.log('Before update: count is', count);
  });

  afterUpdate(() => {
    console.log('After update: count is', count);
  });

  onDestroy(() => {
    console.log('Component destroyed');
  });

  function increment() {
    count += 1;
  }
</script>

<button on:click={increment} aria-label="Increment count">Increment</button>
<p>Count: {count}</p>

Important Notes

Hooks run in the order you add them, so plan your middleware carefully.

Use onDestroy to clean up timers or subscriptions to avoid memory leaks.

Middleware hooks help keep your component code clean and focused on UI.

Summary

Middleware hooks let you run code at key moments in a component's life.

Use onMount, beforeUpdate, afterUpdate, and onDestroy for middleware patterns.

This helps add features like logging, validation, and cleanup in a neat way.

Practice

1. Which Svelte hook is best suited to run code right after a component appears on the page?

easy

A. onDestroy

B. onMount

C. afterUpdate

D. beforeUpdate

Solution

1. Step 1: Understand when onMount runs

   onMount runs once right after the component is added to the page.

2. Step 2: Compare with other hooks

   beforeUpdate and afterUpdate run on updates, onDestroy runs when component is removed.

3. Final Answer:

   onMount -> Option B

4. Quick Check:

   Run code after component appears = onMount [OK]

Hint: Use onMount for code after component loads [OK]

Common Mistakes:

• Confusing onMount with beforeUpdate
• Using onDestroy to run code on load
• Thinking afterUpdate runs only once

2. Which is the correct syntax to import and use the onDestroy hook in a Svelte component?

easy

A. import { onDestroy } from 'svelte/hooks'; onDestroy(() => { /* cleanup */ });

B. import onDestroy from 'svelte'; onDestroy(() => { /* cleanup */ });

C. import { onDestroy } from 'svelte'; onDestroy = () => { /* cleanup */ };

D. import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ });

Solution

1. Step 1: Check correct import syntax

   Svelte exports hooks like onDestroy as named exports from 'svelte'.

2. Step 2: Verify usage

   The hook is called as a function with a callback: onDestroy(() => { ... }).

3. Final Answer:

   import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); -> Option D

4. Quick Check:

   Named import + callback function = import { onDestroy } from 'svelte'; onDestroy(() => { /* cleanup */ }); [OK]

Hint: Use named import from 'svelte' and call with callback [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong path like 'svelte/hooks'
• Assigning hook to a variable instead of calling it

3. Given this Svelte code snippet, what will be logged to the console when the component updates?

import { beforeUpdate, afterUpdate } from 'svelte';

beforeUpdate(() => console.log('Before update'));
afterUpdate(() => console.log('After update'));

let count = 0;

function increment() {
  count += 1;
}

medium

A. After update only once after first render

B. Only Before update once when component mounts

C. Before update\nAfter update each time increment() is called

D. No logs because hooks are not called

Solution

1. Step 1: Understand when hooks run

   beforeUpdate runs before every DOM update, afterUpdate runs after every DOM update.

2. Step 2: Analyze increment effect

   Calling increment() changes count, triggering component update and both hooks logging messages.

3. Final Answer:

   Before update\nAfter update each time increment() is called -> Option C

4. Quick Check:

   Hooks run on every update = Before update\nAfter update each time increment() is called [OK]

Hint: Hooks run before and after every update triggered by state change [OK]

Common Mistakes:

• Thinking hooks run only once on mount
• Assuming logs happen without calling increment()
• Confusing beforeUpdate and afterUpdate timing

4. What is wrong with this Svelte code using onDestroy?

import { onDestroy } from 'svelte';

onDestroy = () => {
  console.log('Cleanup');
};

medium

A. Assigning a function to onDestroy instead of calling it

B. Missing import statement for onDestroy

C. Using arrow function instead of regular function

D. onDestroy should be used inside onMount

Solution

1. Step 1: Check how onDestroy is used

   The code assigns a function to onDestroy instead of calling it with a callback.

2. Step 2: Correct usage

   onDestroy must be called like onDestroy(() => { ... }) to register cleanup.

3. Final Answer:

   Assigning a function to onDestroy instead of calling it -> Option A

4. Quick Check:

   Use onDestroy(callback), not assignment [OK]

Hint: Call onDestroy with a callback, don't assign it [OK]

Common Mistakes:

• Assigning instead of calling the hook
• Thinking arrow functions are disallowed
• Believing onDestroy must be inside onMount

5. You want to log a message every time a Svelte component updates but only after the DOM has changed. Which hook should you use and why? Also, how would you ensure cleanup if needed?

hard

A. Use afterUpdate to log after DOM changes; use onDestroy for cleanup

B. Use beforeUpdate to log after DOM changes; no cleanup needed

C. Use onMount to log after DOM changes; cleanup inside afterUpdate

D. Use onDestroy to log after DOM changes; cleanup inside beforeUpdate

Solution

1. Step 1: Identify hook for after DOM update

   afterUpdate runs after the DOM updates, perfect for logging post-change.

2. Step 2: Determine cleanup hook

   onDestroy runs when component is removed, ideal for cleanup tasks.

3. Final Answer:

   Use afterUpdate to log after DOM changes; use onDestroy for cleanup -> Option A

4. Quick Check:

   Log after DOM update = afterUpdate; cleanup = onDestroy [OK]

Hint: Log in afterUpdate; cleanup in onDestroy [OK]

Common Mistakes:

• Using beforeUpdate to log after DOM changes
• Trying to cleanup inside afterUpdate
• Confusing onMount with update hooks