Switch newsletter form to SvelteKit Form Actions

- remove api endpoint and use latest SK form actions
- create reusable server page function for form handling
- handling the errors in the server function

src/utils/forms/subscribe.ts

@@ -0,0 +1,49 @@
+import { invalid } from '@sveltejs/kit'
+import { NEWSLETTER_API_TOKEN, NEWSLETTER_LIST_ID } from '$env/static/private'
+
+const formMessages = {
+    PENDING: `Almost there! Please confirm your email address through the email you'll receive soon.`,
+    MEMBER_EXISTS_WITH_EMAIL_ADDRESS: `This email address is already subscribed to the newsletter.`,
+    INVALID_EMAIL: `Woops. This email doesn't seem to be valid.`,
+}
+
+
+export default async ({ request }) => {
+    const formData = await request.formData()
+    const email = formData.get('email')
+
+    // Newsletter API request
+    const req = await fetch(`https://emailoctopus.com/api/1.6/lists/${NEWSLETTER_LIST_ID}/contacts`, {
+        method: 'POST',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({
+            api_key: NEWSLETTER_API_TOKEN,
+            email_address: email,
+        })
+    })
+
+    const res = await req.json()
+
+    if (res) {
+        if (res && res.status !== 'PENDING') {
+            // Invalid email
+            if (!email.match(/^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+@[a-zA-Z0-9-]+(?:\.[a-zA-Z0-9-]+)*$/)) {
+                return invalid(400, {
+                    error: 'INVALID_EMAIL',
+                    message () { return formMessages[this.error] }
+                })
+            }
+
+            // Other error
+            return invalid(400, {
+                error: res.error.code,
+                message: formMessages[res.error.code],
+            })
+        }
+
+        return {
+            success: true,
+            message: formMessages[res.status],
+        }
+    }
+}