[adapter-vercel] don't use top level await in Vercel Edge Functions

[Schniz]

This fixes a regression created in #6327, due to the usage of top level await, which isn't supported in Vercel Edge Functions right now.
Using a deferred promise and awaiting within the response it practically the same thing, but works.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 5955b60

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 3 packages

Name	Type
@sveltejs/adapter-vercel	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-auto	Patch

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[dummdidumm]

Thank you!

[evdama]

The problem still exists with "svelte": "^5.0.0-next.167", "@sveltejs/kit": "^2.5.18" and "@sveltejs/adapter-vercel": "^5.4.0" using runtime: edge

// vite.config.ts

import { defineConfig } from 'vitest/config'
import { sveltekit } from '@sveltejs/kit/vite'
export default defineConfig({
  build: { target: 'esnext' },
	plugins: [sveltekit()],
	test: {
		include: ['tests/vitest/**/*.{test,spec}.{js,ts}']
	}
})

// svelte.config.js

/** @type {import('@sveltejs/kit').Config} */
import adapter from '@sveltejs/adapter-vercel'
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte'

const config = {
  kit: {
    adapter: adapter( { runtime: 'edge' } ),   // <---- edge functions
    alias: {
      $components: 'src/components',
    },
  },
  preprocess: vitePreprocess(),
  vitePlugin: {inspector: true}
}

export default config

The top-level await is part of my firebase Initialization

// src/lib/firebase/client-init.ts

import { getAuth, type Auth } from 'firebase/auth'
import { deleteApp, getApp, getApps, initializeApp, type FirebaseApp, type FirebaseOptions } from 'firebase/app'
import { getFirestore, type Firestore } from 'firebase/firestore'

import {
  PUBLIC_FIREBASE_APIKEY,
  PUBLIC_FIREBASE_APPID,
  PUBLIC_FIREBASE_AUTHDOMAIN,
  PUBLIC_FIREBASE_MEASUREMENTID,
  PUBLIC_FIREBASE_MESSAGINGSENDERID,
  PUBLIC_FIREBASE_PROJECTID,
  PUBLIC_FIREBASE_STORAGEBUCKET
} from '$env/static/public'

const firebaseConfig: FirebaseOptions = {
  apiKey: PUBLIC_FIREBASE_APIKEY,
  appId: PUBLIC_FIREBASE_APPID,
  authDomain: PUBLIC_FIREBASE_AUTHDOMAIN,
  measurementId: PUBLIC_FIREBASE_MEASUREMENTID,
  messagingSenderId: PUBLIC_FIREBASE_MESSAGINGSENDERID,
  projectId: PUBLIC_FIREBASE_PROJECTID,
  storageBucket: PUBLIC_FIREBASE_STORAGEBUCKET
}

// REVIEW Switch back to using top-level await; depends on adapter-vercel's edge runtime support
// Make sure we have only one firebaseApp instance
//   This is the correct version as it's using `await deleteApp()` which returns a promise
//    - However Vercel's edge runtime does not support top-level await as of now (June 2024)
//    - see https://github.com/sveltejs/kit/issues/7300
// const firebaseApp: FirebaseApp = (getApps().length === 0 ) ? initializeApp(firebaseConfig) : (await deleteApp(getApp()), initializeApp(firebaseConfig))

// Use `deleteApp()` without await for now
const firebaseApp: FirebaseApp = (getApps().length === 0 ) ? initializeApp(firebaseConfig) : (deleteApp(getApp()), initializeApp(firebaseConfig))

export const firebaseAuth: Auth = getAuth(firebaseApp)
export const firebaseFirestore: Firestore = getFirestore(firebaseApp)