Dealing with errors during form submissions( SvelteKit ).

aymen guendez

Forms. They're the lifeblood of user interaction. From logging in to ordering that new gadget you've been eyeing, forms are everywhere. But let's be honest, things don't always go according to plan. Users mistype emails, servers hiccup, and sometimes, just sometimes, gremlins sneak into the code.

That's where robust error handling comes in. It's the difference between a frustrated user abandoning your site and a user who feels supported and encouraged to try again. In this blog post, we'll dive into how to gracefully handle errors during form submissions in SvelteKit, ensuring a smooth and user-friendly experience, even when things go wrong.

The Anatomy of a SvelteKit Form

Before we tackle errors, let's quickly recap the basic structure of a SvelteKit form. You'll typically have:

  • A Svelte component: This houses your form elements (inputs, textareas, buttons, etc.) and the logic to handle user input.
  • A +page.server.ts file: This is where the server-side magic happens. It defines your form actions, which process the submitted data.

The Common Culprits: Where Errors Lurk

Errors during form submissions can arise from various sources:

  • Client-side validation: Incorrect data format (e.g., invalid email, missing required fields).
  • Server-side validation: More complex validation rules, like checking data against a database.
  • Server-side errors: Database connection issues, API failures, or unexpected code exceptions.

The SvelteKit Way: Handling Errors Gracefully

SvelteKit provides several mechanisms to deal with these errors:

1. Client-Side Validation: Preventing Errors Before They Happen

Client-side validation offers immediate feedback to the user, preventing unnecessary server requests. You can use plain JavaScript, but libraries like Yup or Zod make validation more declarative and maintainable.

<script>
  import { yupResolver } from '@hookform/resolvers/yup';
  import { useForm } from 'svelte-use-form';
  import * as yup from 'yup';

  const schema = yup.object().shape({
    email: yup.string().email('Invalid email').required('Email is required'),
    password: yup.string().min(8, 'Password must be at least 8 characters').required('Password is required'),
  });

  const { form, errors, handleSubmit } = useForm({
    resolver: yupResolver(schema),
  });

  const onSubmit = async (data) => {
    // Form is valid, send data to the server
    console.log('Form data:', data);
    // ... Your submission logic here
  };
</script>

<form use:form on:submit|preventDefault={handleSubmit(onSubmit)}>
  <label for="email">Email:</label>
  <input type="email" id="email" name="email" />
  {#if $errors.email}
    <span class="error">{$errors.email.message}</span>
  {/if}

  <label for="password">Password:</label>
  <input type="password" id="password" name="password" />
  {#if $errors.password}
    <span class="error">{$errors.password.message}</span>
  {/if}

  <button type="submit">Submit</button>
</form>

<style>
  .error {
    color: red;
  }
</style>

In this example:

  • We use svelte-use-form along with yup for validation.
  • The schema defines validation rules for email and password.
  • The $errors store contains error messages, which are displayed dynamically.

2. Server-Side Validation: The Last Line of Defense

Never rely solely on client-side validation! Always validate data on the server to ensure security and data integrity. This is where your +page.server.ts comes into play.

// src/routes/+page.server.ts
import { fail } from '@sveltejs/kit';

export const actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    const email = data.get('email');
    const password = data.get('password');

    if (!email) {
      return fail(400, { emailMissing: true, message: 'Email is required' });
    }

    if (typeof email !== 'string' || !email.includes('@')) {
      return fail(400, { emailInvalid: true, message: 'Invalid email format' });
    }

    if (!password) {
        return fail(400, { passwordMissing: true, message: 'Password is required' });
    }

    // Here you can add database check and more robust validation logic.

    // Simulate a server-side error
    // throw new Error('Something went wrong on the server.');

    // If everything is valid, process the data
    console.log('Server-side data processing', { email, password });
    return { success: true, message: 'Form submitted successfully!' };
  },
};
  • The fail function is crucial for returning errors. It takes an HTTP status code and a data object, which is then passed back to the client-side component.
  • We check for required fields and email format. You can add more complex validation here.
  • Important: We're using fail(400, ...) to send back a 400 Bad Request error. This is good practice for informing the client that their request was invalid.
  • The returned data is merged with the existing data prop, making it available in your Svelte component.

3. Handling Server-Side Errors: Unexpected Hiccups

Sometimes, things go wrong on the server that you can't anticipate. Wrap your form processing logic in a try...catch block to catch exceptions.

// src/routes/+page.server.ts
import { fail } from '@sveltejs/kit';

export const actions = {
    default: async ({ request }) => {
        try {
            const data = await request.formData();
            const email = data.get('email');
            const password = data.get('password');

            if (!email) {
                return fail(400, { emailMissing: true, message: 'Email is required' });
            }

            if (typeof email !== 'string' || !email.includes('@')) {
                return fail(400, { emailInvalid: true, message: 'Invalid email format' });
            }

            if (!password) {
                return fail(400, { passwordMissing: true, message: 'Password is required' });
            }

            // Here you can add database check and more robust validation logic.

            // Simulate a server-side error
            // throw new Error('Something went wrong on the server.');

            // If everything is valid, process the data
            console.log('Server-side data processing', { email, password });
            return { success: true, message: 'Form submitted successfully!' };

        } catch (error:any) {
            console.error('Server-side error:', error);
            return fail(500, { message: 'An unexpected error occurred. Please try again later.' });
        }
    },
};

4. Displaying Errors in Your Svelte Component

Back in your Svelte component, you can access the error data returned by the server. You'll need to ensure you're using the enhance action from @sveltejs/kit on your form, which automatically handles data updates on form submission.

<script>
    import { enhance } from '$app/forms';
    import { page } from '$app/stores';

    $: emailMissing = $page.data?.emailMissing || false;
    $: emailInvalid = $page.data?.emailInvalid || false;
    $: passwordMissing = $page.data?.passwordMissing || false;
    $: serverErrorMessage = $page.data?.message;


    let submitting = false;
</script>

<form method="POST" use:enhance={() => {
    submitting = true;
    return ({ result }) => {
        submitting = false;
    };
}}>
    <label for="email">Email:</label>
    <input type="email" id="email" name="email" />
    {#if emailMissing}
        <span class="error">Email is required.</span>
    {/if}
    {#if emailInvalid}
        <span class="error">Invalid email format.</span>
    {/if}

    <label for="password">Password:</label>
    <input type="password" id="password" name="password" />
    {#if passwordMissing}
        <span class="error">Password is required.</span>
    {/if}

    {#if serverErrorMessage}
        <span class="error">{serverErrorMessage}</span>
    {/if}

    <button type="submit" disabled={submitting}>
      {#if submitting}Submitting...{:else}Submit{/if}
    </button>
</form>

<style>
  .error {
    color: red;
  }
</style>
  • We import enhance from $app/forms and apply it to the <form> element. This function manages the form submission, including updating the page store with the server's response.
  • We access the error data from the $page.data store. Notice the use of the optional chaining operator (?.) to prevent errors if the data isn't present.
  • We conditionally render error messages based on the flags received from the server.
  • We use a submitting flag to disable the button during form submission, improving the user experience.

Best Practices for Error Handling in SvelteKit Forms:

  • Prioritize User Experience: Provide clear and helpful error messages. Avoid technical jargon. Tell the user what went wrong and, ideally, how to fix it.
  • Client-Side First: Implement client-side validation to catch common errors quickly.
  • Server-Side is King: Always validate data on the server to ensure security and data integrity.
  • Handle Exceptions: Use try...catch blocks to gracefully handle unexpected server-side errors.
  • Use the fail Function: SvelteKit's fail function is your best friend for returning errors from your server actions.
  • Log Errors: Log server-side errors for debugging and monitoring.
  • Test Thoroughly: Test your forms with different types of input and simulate server errors to ensure your error handling is robust.
  • Show a Loading state: Disable the button while the form is submitting to prevent double-clicking.

Conclusion

Handling errors effectively is paramount for creating a positive user experience. By implementing client-side validation, robust server-side logic, and clear error messages, you can ensure that your SvelteKit forms are resilient and user-friendly, even when things don't go according to plan. So go forth, build amazing forms, and remember, a little error handling goes a long way!

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