Add a loading indicator to a Form Action in SvelteKit

In order to implement a loading indicator in a SvelteKit form using use:enhance Form Actions, you need to customize use:enhance.

The basic idea is:

1. Create a sending boolean variable that represents whether the form is submitting or not
2. Customize use enhance and set sending to true when the form starts loading, and then to false when the form finishes.

Let’s look at an example of a component where you send in your name and it returns back “Hello ${name}!”.

<script>
    import { enhance } from "$app/forms";
    import { invalidateAll } from "$app/navigation";
    export let form;
    let sending = false;
</script>

<h1>Greeter</h1>

{#if sending}
    <p>
        Sending...
    </p>
{:else if form}
    <p>{form?.greeting}</p>
{:else}
    <p>What's your name?</p>
{/if}

<form
    method="POST"
    use:enhance={() => {
        sending = true;
        return ({ update }) => {
            // Set invalidateAll to false if you don't want to reload page data when submitting
            update({ invalidateAll: true }).finally(async () => { 
                sending = false;
            });
        };
    }}
>
    <input disabled={sending} type="text" name="name" autofocus />
    <button disabled={sending} type="submit">Submit</button>
</form>

And here is the Form Action:

import type { Actions } from './$types';

export const actions = {
    default: async ({ request }) => {
        const data = await request.formData();
        const text = data.get('name');
        await new Promise((resolve) => setTimeout(resolve, 3000));
        return {
            greeting: `Hello ${text}!`,
        }
    },
} satisfies Actions;

Some CSS was ommitted. With this component, we get he below effect. You can also check out the live demo here!