enchance forms in SvelteKit ( compiled TS source from https://github.com/sveltejs/kit/blob/master/packages/create-svelte/templates/default/src/lib/form.ts )

form.js

import { invalidate } from '$app/navigation';
// this action (https://svelte.dev/tutorial/actions) allows us to
// progressively enhance a <form> that already works without JS
/**
 * @param {HTMLFormElement} form
 * @param {{
 *   pending?: ({ data, form }: { data: FormData; form: HTMLFormElement }) => void;
 *   error?: ({
 *     data,
 *     form,
 *     response,
 *     error
 *   }: {
 *     data: FormData;
 *     form: HTMLFormElement;
 *     response: Response | null;
 *     error: Error | null;
 *   }) => void;
 *   result?: ({
 *     data,
 *     form,
 *     response
 *   }: {
 *     data: FormData;
 *     response: Response;
 *     form: HTMLFormElement;
 *   }) => void;
 * }} [opts]
 */
export function enhance(form, { pending, error, result } = {}) {
    let current_token;
    /** @param {SubmitEvent} e */
    async function handle_submit(e) {
        const token = (current_token = {});
        e.preventDefault();
        const data = new FormData(form);
        if (pending)
            pending({ data, form });
        try {
            const response = await fetch(form.action, {
                method: form.method,
                headers: {
                    accept: 'application/json'
                },
                body: data
            });
            if (token !== current_token)
                return;
            if (response.ok) {
                if (result)
                    result({ data, form, response });
                const url = new URL(form.action);
                url.search = url.hash = '';
                invalidate(url.href);
            }
            else if (error) {
                error({ data, form, error: null, response });
            }
            else {
                console.error(await response.text());
            }
        }
        catch (e) {
            if (error && e instanceof Error) {
                error({ data, form, error: e, response: null });
            }
            else {
                throw e;
            }
        }
    }
    form.addEventListener('submit', handle_submit);
    return {
        destroy() {
            form.removeEventListener('submit', handle_submit);
        }
    };
}