Debugging

Svelte provides excellent debugging tools and techniques to help you identify and fix issues in your applications.

Svelte DevTools

The official browser extension provides powerful debugging capabilities:

Installation

Chrome/Edge: Svelte DevTools on Chrome Web Store
Firefox: Svelte DevTools on Firefox Add-ons

Features

Component Tree:

View component hierarchy
Inspect component props and state
See which components are mounted
Track component parent-child relationships

State Inspector:

View all reactive state ($state)
Inspect derived values ($derived)
Monitor state changes in real-time
Modify state values directly

Event Timeline:

Track component lifecycle events
See when effects run
Monitor prop updates

Enable Svelte DevTools in development mode for the best debugging experience. It automatically detects Svelte applications.

The `$inspect` Rune

Basic Usage

Log reactive state changes with $inspect:

<script>
  let count = $state(0);
  let user = $state({ name: 'Alice', age: 30 });
  
  // Logs whenever count or user changes
  $inspect(count, user);
</script>

<button onclick={() => count++}>Count: {count}</button>
<button onclick={() => user.age++}>Age: {user.age}</button>

Output:

count: 1
user: { name: 'Alice', age: 31 }
  at Component.svelte:5:2

$inspect only works in development mode. It becomes a no-op in production builds.

Deep Reactivity Tracking

$inspect tracks changes deeply in objects and arrays:

<script>
  let todos = $state([
    { id: 1, text: 'Learn Svelte', done: false },
    { id: 2, text: 'Build app', done: false }
  ]);
  
  $inspect(todos);
  
  function toggleTodo(id) {
    const todo = todos.find(t => t.id === id);
    todo.done = !todo.done; // $inspect logs this mutation
  }
</script>

Custom Logging with `.with`

Provide your own logging function:

<script>
  let count = $state(0);
  
  $inspect(count).with((type, value) => {
    if (type === 'update') {
      console.log(`Count changed to ${value}`);
      debugger; // Pause execution on changes
    }
  });
</script>

Parameters:

type: "init" (first call) or "update" (subsequent changes)
...values: The values passed to $inspect

Tracing Reactive Dependencies

Use $inspect.trace() to debug why effects or derived values re-run:

<script>
  let firstName = $state('Alice');
  let lastName = $state('Smith');
  let age = $state(30);
  
  let greeting = $derived.by(() => {
    $inspect.trace('greeting calculation');
    return `${firstName} ${lastName} (${age})`;
  });
</script>

<input bind:value={firstName} placeholder="First name" />
<input bind:value={lastName} placeholder="Last name" />
<input bind:value={age} type="number" />

<p>{greeting}</p>

Console output when firstName changes:

greeting calculation
Dependencies:
  - firstName (changed)
  - lastName
  - age

$inspect.trace() must be the first statement in an effect or derived function body.

Browser DevTools

Chrome DevTools

Elements Panel:

Inspect DOM structure
View component-generated HTML
Modify styles in real-time
Check element accessibility

Console:

<script>
  let data = $state({ items: [] });
  
  // Make available in console for debugging
  globalThis.debug = { data };
  
  $effect(() => {
    console.log('Data updated:', data);
  });
</script>

Now in console:

> debug.data
{ items: [...] }

> debug.data.items.push({ id: 4 })
// Triggers reactive updates

Performance Panel:

Start recording
Interact with your app (click, scroll, type)
Stop recording
Analyze flame graph for slow operations
Identify expensive component renders

Network Panel:

Monitor API requests
Check request/response times
Debug failed requests
Inspect headers and payloads

Source Maps

Svelte generates source maps automatically in development:

// vite.config.js
export default {
  build: {
    sourcemap: true // Enable in production for debugging
  }
};

With source maps, you can:

Set breakpoints in .svelte files
Step through original source code
See original variable names
Get accurate stack traces

Common Debugging Scenarios

Reactivity Not Working

Problem: Component doesn’t update when state changes

<script>
  // ❌ Bad: Not reactive
  let count = 0;
  
  function increment() {
    count++; // DOM won't update
  }
</script>

<button onclick={increment}>{count}</button>

Solution: Use $state

<script>
  // ✅ Good: Reactive
  let count = $state(0);
  
  function increment() {
    count++; // DOM updates
  }
</script>

Debug with $inspect.trace:

<script>
  let count = $state(0);
  
  $effect(() => {
    $inspect.trace('count effect');
    console.log('Count:', count);
  });
</script>

Props Not Updating

Problem: Child component doesn’t react to prop changes

<!-- Child.svelte -->
<script>
  let { value } = $props();
  
  // ❌ Bad: Captures initial value only
  let doubled = value * 2;
</script>

<p>{doubled}</p>

Solution: Use $derived for computed props

<script>
  let { value } = $props();
  
  // ✅ Good: Recalculates when value changes
  let doubled = $derived(value * 2);
</script>

Infinite Loops in Effects

Problem: Effect triggers itself

<script>
  let items = $state([]);
  
  $effect(() => {
    // ❌ Bad: Modifies its own dependency
    items = [...items, Math.random()];
  });
</script>

Solution: Use the right tool

<script>
  let count = $state(0);
  
  // ✅ For initial setup only
  $effect(() => {
    const interval = setInterval(() => {
      count++; // This is fine - effect doesn't depend on count
    }, 1000);
    
    return () => clearInterval(interval);
  });
</script>

Debug infinite loops:

<script>
  let data = $state([]);
  let runCount = 0;
  
  $effect(() => {
    console.log('Effect run #', ++runCount);
    if (runCount > 10) {
      debugger; // Pause after 10 runs
    }
    // ... effect code
  });
</script>

Component Not Rendering

Problem: Component instance doesn’t appear in DOM Check:

Import path: Is the component imported correctly?
Component name: Does it start with a capital letter?
Conditional rendering: Is it inside a falsy {#if} block?
Parent mount: Is the parent component mounted?

<script>
  import MyComponent from './MyComponent.svelte'; // Check path
  
  let show = $state(true);
  
  $effect(() => {
    console.log('Parent mounted');
  });
</script>

{#if show}
  <MyComponent /> <!-- Capital letter required -->
{:else}
  <p>Component hidden</p>
{/if}

Error Boundaries

Catch and handle component errors gracefully:

<script>
  import { ErrorBoundary } from './ErrorBoundary.svelte';
  import RiskyComponent from './RiskyComponent.svelte';
</script>

<svelte:boundary onerror={(error) => console.error(error)}>
  <RiskyComponent />
  
  {#snippet failed(error)}
    <div class="error">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
    </div>
  {/snippet}
</svelte:boundary>

Use <svelte:boundary> to prevent errors from crashing your entire app.

Testing and Debugging

Component Tests with Vitest

Write tests that act as debugging documentation:

import { render, screen } from '@testing-library/svelte';
import { expect, test } from 'vitest';
import Counter from './Counter.svelte';

test('counter increments', async () => {
  const { component } = render(Counter, { props: { initial: 0 } });
  
  const button = screen.getByRole('button');
  expect(button).toHaveTextContent('0');
  
  await button.click();
  expect(button).toHaveTextContent('1');
});

Debug Mode in Tests

import { render } from '@testing-library/svelte';
import { expect, test } from 'vitest';
import Component from './Component.svelte';

test.only('debug specific test', () => {
  const { debug } = render(Component);
  
  debug(); // Prints current DOM
  
  // ... test assertions
});

Logging Best Practices

Structured Logging

<script>
  function fetchData() {
    console.group('fetchData');
    console.time('fetch-duration');
    
    fetch('/api/data')
      .then(r => r.json())
      .then(data => {
        console.log('Data:', data);
        console.timeEnd('fetch-duration');
        console.groupEnd();
      })
      .catch(error => {
        console.error('Fetch failed:', error);
        console.groupEnd();
      });
  }
</script>

Conditional Logging

<script>
  const DEBUG = import.meta.env.DEV;
  
  function log(...args) {
    if (DEBUG) console.log(...args);
  }
  
  let count = $state(0);
  
  $effect(() => {
    log('Count changed:', count);
  });
</script>

Development vs Production

Environment Detection

<script>
  import { DEV } from 'esm-env';
  
  if (DEV) {
    console.log('Development mode');
    globalThis.debugApp = {
      // Expose debugging utilities
    };
  }
</script>

Debug Panels

<script>
  import { DEV } from 'esm-env';
  import DebugPanel from './DebugPanel.svelte';
  
  let showDebug = $state(false);
</script>

{#if DEV}
  <button onclick={() => showDebug = !showDebug}>
    Toggle Debug Panel
  </button>
  
  {#if showDebug}
    <DebugPanel />
  {/if}
{/if}

Debugging Tips

Use $inspect liberally - It’s removed in production automatically
Enable Svelte DevTools - Essential for understanding component state
Add debugger statements - Pause execution at specific points
Log component lifecycle - Track when components mount/unmount
Test edge cases - Write tests for problematic scenarios
Use TypeScript - Catch type errors before runtime
Check the compiler warnings - Svelte warns about common mistakes
Profile performance - Use browser tools to find bottlenecks

Common mistakes to avoid:

Forgetting to use $state for reactive variables
Using $effect instead of $derived for computed values
Not providing keys in {#each} blocks
Modifying props directly (use callbacks or events)
Ignoring compiler warnings

Additional Resources

The best debugging tool is prevention. Use TypeScript, write tests, and pay attention to compiler warnings to catch issues early.

Advanced Topics

Migration

Svelte DevTools

Installation

Features

The `$inspect` Rune

Basic Usage

Deep Reactivity Tracking

Custom Logging with `.with`

Tracing Reactive Dependencies

Browser DevTools

Chrome DevTools

Source Maps

Common Debugging Scenarios

Reactivity Not Working

Props Not Updating

Infinite Loops in Effects

Component Not Rendering

Error Boundaries

Testing and Debugging

Component Tests with Vitest

Debug Mode in Tests

Logging Best Practices

Structured Logging

Conditional Logging

Development vs Production

Environment Detection

Debug Panels

Debugging Tips

Additional Resources

Advanced Topics

Migration

​Svelte DevTools

​Installation

​Features

​The $inspect Rune

​Basic Usage

​Deep Reactivity Tracking

​Custom Logging with .with

​Tracing Reactive Dependencies

​Browser DevTools

​Chrome DevTools

​Source Maps

​Common Debugging Scenarios

​Reactivity Not Working

​Props Not Updating

​Infinite Loops in Effects

​Component Not Rendering

​Error Boundaries

​Testing and Debugging

​Component Tests with Vitest

​Debug Mode in Tests

​Logging Best Practices

​Structured Logging

​Conditional Logging

​Development vs Production

​Environment Detection

​Debug Panels

​Debugging Tips

​Additional Resources

Svelte DevTools

Installation

Features

The `$inspect` Rune

Basic Usage

Deep Reactivity Tracking

Custom Logging with `.with`

Tracing Reactive Dependencies

Browser DevTools

Chrome DevTools

Source Maps

Common Debugging Scenarios

Reactivity Not Working

Props Not Updating

Infinite Loops in Effects

Component Not Rendering

Error Boundaries

Testing and Debugging

Component Tests with Vitest

Debug Mode in Tests

Logging Best Practices

Structured Logging

Conditional Logging

Development vs Production

Environment Detection

Debug Panels

Debugging Tips

Additional Resources