Handling Validation Errors in Actions
Return validation errors and repopulate form fields using fail() and ActionData.
Handling Validation Errors in Actions is a free Sveltejs Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Sveltejs Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The fail Helper
Use fail() to return validation errors with a non-2xx status without throwing.
import { fail } from "@sveltejs/kit";
if (!email) return fail(400, { email, missing: true });Status Code
First argument is the HTTP status (usually 400 for validation).
Error Payload
The second argument is data sent back to the page. Include the user's input for repopulation.
Read in Page
The page receives a form prop with the action result.
<script>
export let form;
</script>
{#if form?.missing}<p>Please enter an email.</p>{/if}Repopulate Fields
Pass the submitted value back via fail and prefill inputs to avoid retyping.
<input name="email" value={form?.email ?? ""} />Field-Level Errors
Use individual error keys per field for granular UI.
return fail(400, { errors: { email: "Invalid" } });Combine With Schema Validation
Use libraries like Zod or Valibot on the server for type-safe parsing.
Avoid Throwing for Validation
Throwing leads to error pages; fail keeps the user on the form.
Form Reset on Success
On success, clear form state by returning empty values or redirecting.
Use With use:enhance
The form prop updates without a reload thanks to enhance.
Accessibility
Announce errors with aria-live so screen readers read them.
Quick Check
What does fail() return?
Recap
Use fail() for validation errors. The page reads them from the form prop for friendly feedback.
Frequently asked questions
Is the “Handling Validation Errors in Actions” lesson free?
Yes — the full text of “Handling Validation Errors in Actions” is free to read here on the web, and the Sveltejs Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Sveltejs Academy course, upgrade to CoddyKit PRO.
What will I learn in “Handling Validation Errors in Actions”?
Return validation errors and repopulate form fields using fail() and ActionData. You practise Sveltejs Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Sveltejs Academy?
No prior experience is required. Sveltejs Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Handling Validation Errors in Actions” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Sveltejs Academy lesson?
Yes. Every Sveltejs Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- +page.server.js with actions
- The use:enhance Directive
- Progressive Enhancement for Forms
- Handling Validation Errors in Actions