website/routes/api/webpush/subscription.ts

import { Handlers } from '$fresh/server.ts'
import { respondApi } from ':src/utils.ts'

export const handler: Handlers = {
	async POST(request: Request) {
		const subscription = await request.json() as PushSubscriptionJSON
		saveSubscription(subscription)
		return respondApi('success', 'ok')
	},
}

function saveSubscription(subscription: PushSubscriptionJSON) {
	const itemKey = 'webpush-subscription'
	const subscriptions = JSON.parse(
		localStorage.getItem(itemKey) ?? '[]',
	) as PushSubscriptionJSON[]

	// Prevent duplicate
	const auth = subscription.keys?.auth
	if (subscriptions.some((sub) => sub.keys?.auth === auth)) {
		return
	}

	// Store subscription
	subscriptions.push(subscription)
	localStorage.setItem(itemKey, JSON.stringify(subscriptions))
}