Error handling in load in Svelte - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the load function in SvelteKit?

The load function fetches data before a page or component renders, allowing you to prepare data or handle errors early.

beginner

How can you handle errors inside the load function in SvelteKit?

You can throw an Error or use the error() helper to send an error status and message, which SvelteKit catches to show error pages.

beginner

What happens if you throw an error inside the load function?

SvelteKit stops loading the page and shows the error page with the status and message you provided.

intermediate

How do you use the error() helper in the load function?

Call throw error(statusCode, message) inside load to send a specific HTTP status and message for the error page.

beginner

Why is error handling in load important for user experience?

It lets you show friendly messages or fallback content instead of broken pages, keeping users informed and the app stable.

What does the load function in SvelteKit primarily do?

A Manage routing between pages

B Handle user input events

C Style components dynamically

D Fetch data before rendering a page

The load function is used to fetch or prepare data before the page renders.

How do you signal an error inside the load function?

A Return null from <code>load</code>

B Throw an error using <code>throw error(status, message)</code>

C Use <code>console.log</code> to print the error

D Call <code>setTimeout</code> with an error

Throwing an error with throw error(status, message) tells SvelteKit to show an error page.

What happens if you don't handle errors in load?

A The page may break or show incomplete data

B The app automatically retries loading

C The error is ignored silently

D The page redirects to home

Without error handling, the page can break or show wrong content, hurting user experience.

Which helper function is used to create HTTP errors in load?

A error()

B fetch()

C redirect()

D json()

The error() helper creates HTTP errors with status codes and messages.

Why should you handle errors in load instead of inside the component?

A Because errors in components crash the browser

B Because components cannot show errors

C Because <code>load</code> runs before rendering and can stop broken pages early

D Because <code>load</code> is faster than components

Handling errors in load stops broken pages before rendering, improving stability.

Explain how to handle errors inside the load function in SvelteKit and why it matters.

Think about how you tell SvelteKit to show an error page from load.

Describe what happens when an error is thrown inside the load function.

Focus on the flow after throwing an error in load.

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