When using Svelte alone, you can use Client-side component API

import App from "./App.svelte"

keycloak.init({ onLoad: initOptions.onLoad }).then((auth) => {
  if (!auth) {
    window.location.reload();
  } else {
    new App({
      target: document.getElementById('#app'),
      props: { keycloak: keycloak } })
    })
  }

In a SvelteKit application, you can add global behavior using src/routes/layout.js:

// layout.js
import { browser } from "$app/environment";

/**
 * @type {import('./$types').LayoutLoad}
 */
export const load = async () => {
  let keycloakPromise;
  if (browser) {
    keycloakPromise = keycloak.init({ onLoad: initOptions.onLoad }).then((auth) => {
      if (auth) {
        return keycloak;
      }
    });
  }
  return {
    keycloak: keycloakPromise,
  };
};

<!-- layout.svelte -->
<script>
  import { onMount } from "svelte";

  /** @type {import('./$types').LayoutData} */
  export let data;

  onMount(() => {
    if (!data.keycloak) {
      window.location.reload();
    }
  });
</script>

{#if data.keycloak}
  <slot />
{/if}

Authentication fails when data.keycloak is not defined. This allows conditional rendering of content.

(code example assumes keycloak is client authentication)