Error handling in load in Svelte - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when load throws an error?

Consider a SvelteKit page with a load function that throws an error. What will the user see by default?

Svelte

export async function load() {
  throw new Error('Failed to load data');
}

A The page shows a default error page with the error message.

B The page silently fails and shows no content.

C The page reloads infinitely until the error is fixed.

D The page shows the previous cached content without error.

💡 Hint

Think about how SvelteKit handles errors thrown in load functions by default.

When a load function throws an error, SvelteKit automatically shows its default error page displaying the error message. This helps users know something went wrong.

📝 Syntax

intermediate

Which load function correctly catches and returns an error?

Which load function properly catches an error and returns a custom error response to the page?

A

export async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) throw new Error('Fetch failed');
    return { data: await res.json() };
  } catch (error) {
    return { status: 500, error: new Error('Custom load error') };
  }
}

B

export async function load() {
  const res = await fetch('/api/data');
  if (!res.ok) return { error: 'Fetch failed' };
  return { data: await res.json() };
}

C

export async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) throw new Error('Fetch failed');
    return { data: await res.json() };
  } catch (error) {
    throw new Error('Custom load error');
  }
}

D

export async function load() {
  try {
    const res = await fetch('/api/data');
    if (!res.ok) throw new Error('Fetch failed');
    return { data: await res.json() };
  } catch (error) {
    return { error: 'Custom load error' };
  }
}

💡 Hint

Remember that to return an error response from load, you must return an object with status and error properties.

Option A correctly catches the error and returns an object with status and error properties, which SvelteKit uses to render an error page with the given status and message.

Option A rethrows the error, which also works but does not return a custom error response object.

Options A and D return an error property without status, which SvelteKit does not interpret as an error response.

🔧 Debug

advanced

Why does this load function not show the error page?

Given this load function, why does the page not show the error page when the fetch fails?

Svelte

export async function load() {
  const res = await fetch('/api/data');
  if (!res.ok) {
    return { error: new Error('Failed to fetch') };
  }
  return { data: await res.json() };
}

A Because the <code>load</code> function must throw the error instead of returning it.

B Because the fetch never fails, so the error block is never reached.

C Because returning an <code>error</code> property without <code>status</code> does not trigger the error page.

D Because the <code>load</code> function must return a rejected promise to trigger the error page.

💡 Hint

Check what SvelteKit expects to show an error page from a load function.

SvelteKit requires the load function to return an object with status and error properties or throw an error to show the error page. Returning only an error property is ignored and treated as normal data.

❓ state_output

advanced

What is the value of error in the page when load returns a custom error?

If the load function returns { status: 404, error: new Error('Not found') }, what will the error prop contain in the error page component?

A A string 'Not found'.

B Undefined, because error is not passed as a prop.

C An object with a <code>message</code> property containing 'Not found'.

D An Error object with message 'Not found'.

💡 Hint

Recall what type of value is passed as error to the error page.

The error prop in the error page receives the actual Error object returned from load. This allows the error page to access the message and stack trace.

🧠 Conceptual

expert

How to handle errors globally in SvelteKit load functions?

Which approach correctly implements global error handling for all load functions in a SvelteKit app?

A Wrap every <code>load</code> function in try-catch and return error objects manually in each page.

B Create a root <code>+error.svelte</code> component to customize error display and throw errors in <code>load</code> functions as needed.

C Use a global <code>hooks.server.js</code> file to catch errors thrown in <code>load</code> functions and return custom responses.

D Set a global variable to track errors and check it in each page's <code>load</code> function.

💡 Hint

Think about SvelteKit's built-in error handling and how to customize error pages.

Option B is correct because SvelteKit uses a root +error.svelte component to display errors globally. Throwing errors in load functions triggers this component. Options A and B are manual and error-prone. Option B is invalid because hooks.server.js does not catch errors from load functions on the client side.