Send an Email in JavaScript

After a payment, the customer expects an email: "we got your money". A shop doesn't run its own mail server. It calls a mail service through its API, like the supplier and the payment provider. Ours lives at http://127.0.0.1:8003: you POST JSON with "to", "subject" and "body" to /send, and it answers 202 Accepted, which means "I have it, I'll deliver it".

The email's text is built from pieces joined with +. When one side is text, JavaScript turns a number on the other side into text as it joins them:

const count = 3;
const text = "You have " + count + " new messages";
console.log(text);   // You have 3 new messages

Put the email in a helper function, so it has its own place and other code can send it too. Make it async, because it waits for the mail service. Call it from the pay route, with await, only once the order is saved as paid: a declined card returns earlier, and so does a retry with the same idempotency key, so neither sends an email. The checker adds GET /check/sent, the list of emails the mail service received:

POST /orders/8/pay  (Idempotency-Key: 5f1d7c9e-…)  →  201  {"amount": 80, "order_id": 8, "payment_id": "pay_1"}
GET /check/sent  →  200  [{"body": "Hello Eva Rossi, we received your payment of 80. Thank you!", "id": 1, "subject": "Your order 8 is paid", "to": "eva@example.com"}]
POST /orders/8/pay  (the same key)  →  201  {"amount": 80, "order_id": 8, "payment_id": "pay_1"}
POST /orders/9/pay  →  402  {"error": "Payment declined", "reason": "card_declined"}
POST /orders/10/pay  →  201  {"amount": 21, "order_id": 10, "payment_id": "pay_2"}
GET /check/sent  →  200  [Eva's email, {"body": "Hello Chloe Ng, we received your payment of 21. Thank you!", "id": 2, "subject": "Your order 10 is paid", "to": "chloe@example.com"}]

Your task: add the constant MAIL, then write the async function sendReceipt(orderId): it finds the order and its customer, builds the subject and the body, and POSTs them to MAIL + "/send", returning the mail service's answer. Call it with await in the pay route, right after the UPDATE.

Assignment
Follow the Coding Tutorial to write the sendReceipt helper and send a receipt after each successful payment.

Hint
The order has a customer_id; the name and the email are in the customers table. Join the pieces of text with +: orderId and the total become text on the way. A function runs only when it's called: until the pay route calls the helper, GET /check/sent stays empty.