Error handling in load in Svelte

Introduction

Error handling in load helps your app manage problems when fetching data or preparing a page. It keeps your app friendly and avoids crashes.

When fetching data from an API that might fail

When loading page data that depends on external sources

When you want to show a friendly message if something goes wrong

When you want to redirect users if loading fails

When you want to log errors for debugging

Syntax

Svelte

export async function load({ fetch, params }) {
  try {
    const response = await fetch('/api/data');
    if (!response.ok) {
      throw new Error('Failed to fetch data');
    }
    const data = await response.json();
    return { data };
  } catch (error) {
    return { status: 500, error: new Error('Could not load data') };
  }
}

The load function runs before the page shows.

Use try...catch to catch errors and return a status and error message.

Examples

This example fetches items and returns a 404 error if it fails.

Svelte

export async function load({ fetch }) {
  try {
    const res = await fetch('/api/items');
    if (!res.ok) throw new Error('Network error');
    const items = await res.json();
    return { items };
  } catch {
    return { status: 404, error: new Error('Items not found') };
  }
}

No error handling needed here because no data is fetched.

Svelte

export async function load() {
  return { message: 'Hello!' };
}

Error is handled by checking response status without try/catch.

Svelte

export async function load({ fetch }) {
  const response = await fetch('/api/data');
  if (!response.ok) {
    return { status: 500, error: new Error('Server error') };
  }
  const data = await response.json();
  return { data };
}

Sample Program

This Svelte page tries to load user data before showing. If loading fails, it shows a friendly message.

Svelte

<script context="module">
  export async function load({ fetch }) {
    try {
      const res = await fetch('/api/user');
      if (!res.ok) {
        throw new Error('User data not found');
      }
      const user = await res.json();
      return { user };
    } catch (error) {
      return { user: null };
    }
  }
</script>

<script>
  export let data;
</script>

<main>
  {#if data.user}
    <h1>Welcome, {data.user.name}!</h1>
  {:else}
    <p>Loading user data failed.</p>
  {/if}
</main>

Important Notes

To trigger SvelteKit's error page, return an object with status and error.

Use try...catch to handle unexpected errors gracefully.

Test your error handling by simulating failed fetch calls in the browser DevTools Network tab.

Summary

Error handling in load keeps your app stable when data fails to load.

Use try...catch and check response status to catch problems.

Return { status, error } to show error pages or messages.

Practice

1. What is the main purpose of using error handling in the load function in SvelteKit?

easy

A. To style the page dynamically

B. To speed up the loading of data

C. To keep the app stable when data fails to load

D. To preload images before rendering

Solution

1. Step 1: Understand the role of load in SvelteKit

   The load function fetches data before the page renders.

2. Step 2: Identify why error handling is needed

   If data fetching fails, error handling prevents the app from crashing and shows a proper message.

3. Final Answer:

   To keep the app stable when data fails to load -> Option C

4. Quick Check:

   Error handling in load = stability [OK]

Hint: Error handling keeps your app stable on data failure [OK]

Common Mistakes:

• Thinking error handling speeds up loading
• Confusing error handling with styling
• Assuming error handling preloads images

2. Which of the following is the correct way to throw an error with status 404 inside a load function?

easy

A. throw new Error(404, 'Not found')

B. throw error(404, 'Not found')

C. return { error: 404, message: 'Not found' }

D. throw { status: 404, message: 'Not found' }

Solution

1. Step 1: Recall SvelteKit's error helper usage

   SvelteKit provides an error function to throw HTTP errors with status and message.

2. Step 2: Match the correct syntax

   The correct syntax is throw error(404, 'Not found'). Other options are invalid or incomplete.

3. Final Answer:

   throw error(404, 'Not found') -> Option B

4. Quick Check:

   Throw error helper = throw error(404, message) [OK]

Hint: Use throw error(status, message) in load [OK]

Common Mistakes:

• Using new Error() with status code
• Returning error object instead of throwing
• Throwing plain objects without error helper

3. Given this load function, what will be the output if the fetch fails with a 500 status?

export async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) {
      throw error(res.status, 'Failed to load data');
    }
    const data = await res.json();
    return { data };
  } catch (err) {
    return { status: err.status || 500, error: err.message };
  }
}

medium

A. { status: 500, error: 'Failed to load data' }

B. { data: undefined }

C. Page crashes with unhandled error

D. { status: 404, error: 'Not found' }

Solution

1. Step 1: Analyze fetch failure handling

   If fetch returns a 500 status, res.ok is false, so throw error(500, 'Failed to load data') runs.

2. Step 2: Understand catch block behavior

   The thrown error is caught, and the function returns an object with status and error from the error.

3. Final Answer:

   { status: 500, error: 'Failed to load data' } -> Option A

4. Quick Check:

   Catch returns error object with status and message [OK]

Hint: Catch returns error object with status and message [OK]

Common Mistakes:

• Expecting data to be returned on fetch failure
• Assuming page crashes without catch
• Confusing 404 with 500 status

4. Identify the error in this load function that causes the error handling to fail:

export async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) {
      return error(res.status, 'Failed to load');
    }
    const data = await res.json();
    return { data };
  } catch (err) {
    return { status: err.status, error: err.message };
  }
}

medium

A. Not checking res.ok before parsing JSON

B. Missing await before fetch

C. Not returning data inside catch block

D. Using return error(...) instead of throw error(...)

Solution

1. Step 1: Check error throwing vs returning

   The code uses return error(...) which does not throw an error but returns a value, so error handling is bypassed.

2. Step 2: Correct usage in SvelteKit

   To trigger error handling, it must throw error(...) so the catch block or error page activates.

3. Final Answer:

   Using return error(...) instead of throw error(...) -> Option D

4. Quick Check:

   Throw error to activate error handling [OK]

Hint: Throw error(...) to trigger error handling, not return [OK]

Common Mistakes:

• Returning error instead of throwing
• Forgetting to await fetch
• Ignoring res.ok check

5. You want to show a custom error page when your API returns a 403 status inside load. Which approach correctly handles this and passes the error to the page?

hard

A. Throw error(403, 'Access denied') inside load and create an error.svelte page to display it

B. Return { status: 403, error: 'Access denied' } and handle it manually in the page script

C. Use console.error and return empty data to show default page

D. Redirect to a different page using goto('/login') inside load

Solution

1. Step 1: Understand error propagation in load

   Throwing error(403, 'Access denied') inside load tells SvelteKit to show the error page.

2. Step 2: Use error.svelte to customize error display

   Creating an error.svelte page lets you show custom messages based on the error status and message.

3. Final Answer:

   Throw error(403, 'Access denied') inside load and create an error.svelte page to display it -> Option A

4. Quick Check:

   Throw error + error.svelte = custom error page [OK]

Hint: Throw error and use error.svelte for custom error pages [OK]

Common Mistakes:

• Returning error object instead of throwing
• Using console.error without error propagation
• Redirecting instead of showing error page