Resolving issues with route-level onError hooks( SvelteKit ).

aymen guendez March 27, 2025

SvelteKit's onError hooks are a powerful tool for handling errors gracefully and providing a better user experience. They allow you to intercept errors that occur during server-side rendering and data loading (loads) at various levels: root, layout, and route. Today, we're diving deep into resolving common issues you might encounter when working with route-level onError hooks.

Let's face it, error handling isn't the most glamorous part of development, but it's essential for building robust and user-friendly applications. Understanding how to properly implement and debug your onError hooks can save you headaches down the line.

What are Route-Level onError Hooks?

Located in your +error.svelte file within a specific route directory, these hooks are specifically designed to handle errors that originate from that route's +page.server.js or +page.js load functions. They provide a way to customize the error page displayed to the user.

Common Issues and How to Resolve Them:

Here are some typical problems you might run into with route-level onError hooks and how to address them:

1. Error Not Being Caught:

Problem: You expect an error from your load function to be caught by your route-level onError hook, but instead, you're seeing a generic error page or the error is bubbling up to a higher-level onError hook.
Solutions:
- Ensure the Error is Thrown: Double-check that your load function is actually throwing an error. Use throw new Error("My Error") or throw error (if you're already catching an error). Simple console.error() statements won't trigger the hook.
```
// +page.server.js
export const load = async () => {
try {
  const data = await fetchData();
  return { data };
} catch (error) {
  throw error; // This is crucial!
}
};
```
- Verify Correct File Placement: The +error.svelte file containing your onError hook must be in the correct directory - the same directory as your +page.svelte or +page.server.js where the error originates. A misplaced file will be ignored.
- Check Hook Syntax: The +error.svelte file should export an onError function that accepts the event as an argument.
```

<script>
/** @type {import('./$types').PageData} */
export let data;
/** @type {import('./$types').OnError} */
export let error;
</script>

<h1>Error!</h1>
<p>{error.message}</p>
<p>Details: {JSON.stringify(data)}</p>
```
- Prioritize Error Handling: If you have multiple onError hooks (root, layout, route), SvelteKit will prioritize the most specific one. Make sure the route-level hook is actually the intended handler.

2. Unexpected Data in the event Parameter:

Problem: You're expecting certain data within the event object in your onError hook, but it's missing or undefined.
Solutions:
- Understand the event Object: The event object passed to the onError hook contains valuable information, including:
  - params: Route parameters.
  - routeId: The ID of the current route.
  - url: The current URL.
  - locals: Any server-side locals you've defined in your hooks.server.js.
- Debug with console.log: The simplest way to troubleshoot is to console.log(event) within your onError hook to inspect its contents. This will help you understand what data is available in that specific context.
```

<script>
/** @type {import('./$types').OnError} */
export let error;
import { onMount } from 'svelte';

onMount(() => {
  console.log('Error Event:', error);
});
</script>

<h1>Error!</h1>
<p>{error.message}</p>
```
- Ensure Data is Available: If you're relying on data loaded in a layout, make sure that layout is applied to the route where the error occurred.

3. Difficulty Customizing the Error Page:

Problem: You want to display specific information or implement custom logic within your error page, but you're struggling to access the necessary data.

Solutions:

Utilize the data Prop: Your +error.svelte file receives a data prop. This data will contain a status (HTTP status code) and message (error message). You can customize the page based on these values.

<!-- +error.svelte -->
<script>
/** @type {import('./$types').PageData} */
export let data;
</script>

{#if data.status === 404}
<h1>Page Not Found</h1>
<p>Sorry, the page you requested could not be found.</p>
{:else}
<h1>Error!</h1>
<p>{data.message}</p>
{/if}

Leverage the error Prop: The error prop provides more detailed information about the error, including the error message and stack trace (in development mode). Be cautious about displaying sensitive information to users in production.

Pass Custom Data: You can modify the data that's ultimately available within the error prop by using the fail function in your form actions. fail lets you return custom data alongside a status code, making it easier to show user-friendly error messages or relevant information. For example:

// +page.server.js
import { fail } from '@sveltejs/kit';

export const actions = {
  default: async ({ request }) => {
      // some validation error happens
      return fail(400, {
          message: "Please enter a valid email address."
      });
  }
};

<!-- +error.svelte -->
<script>
/** @type {import('./$types').PageData} */
export let data;
</script>
{#if data?.form?.message}
  <p>Custom Form Error: {data.form.message}</p>
{/if}

4. Incorrect Error Handling Logic:

Problem: The error handling logic itself in your onError hook is flawed, causing unexpected behavior or infinite loops.
Solutions:
- Carefully Design Your Error Handling: Think critically about what you want to happen when an error occurs. Do you need to redirect the user? Display a specific message? Attempt to retry the operation?
- Avoid Infinite Loops: Be extremely careful not to create an infinite loop within your error handling logic. For instance, repeatedly calling a failing load function will trigger the onError hook again and again. Consider using a flag to prevent retry attempts after a certain threshold.
- Use try...catch Blocks Strategically: Employ try...catch blocks within your onError hook if necessary to handle potential errors that might occur within the error handling process itself.

Tips for Debugging onError Hooks:

Use the Browser Developer Tools: Utilize the browser's developer tools (especially the console) to inspect errors and log data.
Set Breakpoints: Place breakpoints in your onError hook to step through the code and examine the variables at runtime.
Temporary console.log Statements: Add console.log statements liberally to track the flow of execution and inspect data.
Start Small: Test your onError hooks with simple error scenarios before tackling more complex ones.
Read the SvelteKit Documentation: The SvelteKit documentation is a fantastic resource for understanding onError hooks and other error handling features.

Example: A Route-Level onError Hook with Custom Data

Here's a complete example to illustrate a route-level onError hook:

// src/routes/products/[id]/+page.server.js
export const load = async ({ params }) => {
  const productId = params.id;

  if (productId === "error") {
    throw new Error("Product not found!");
  }

  // Simulate fetching product data
  const product = {
    id: productId,
    name: `Product ${productId}`,
    description: `Description of Product ${productId}`,
  };

  return { product };
};

<!-- src/routes/products/[id]/+error.svelte -->
<script>
  /** @type {import('./$types').PageData} */
  export let data;
  /** @type {import('./$types').OnError} */
  export let error;
</script>

<h1>Product Error</h1>
<p>
  There was an error fetching the product: <strong>{error.message}</strong>
</p>

{#if error.message === "Product not found!"}
  <p>Please check the product ID.</p>
{/if}

Conclusion:

Route-level onError hooks are an essential tool for building robust and user-friendly SvelteKit applications. By understanding common issues and following the debugging tips outlined in this guide, you can effectively handle errors and provide a great experience for your users, even when things go wrong. Remember to test your error handling thoroughly and tailor it to the specific needs of your application. Good luck!

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