TypeError: Cannot read properties of undefined (reading 'map') - why is map not defined?

Question

Can someone explain to me why 'map' is undefined? I provided my server.js and checkout.js below

The problem is that when I click a product to add to the cart to checkout it shows all the products, including the prices and total, on the stripe session instead of the item I selected. My goal is to only show one of the items selected from the actual website

server.js

//require("dotenv").config()

// This is your test secret API key.
const stripe = require("stripe")(
  "sk_test_XXXXXX"
);

const express = require("express");
const app = express();
app.use(express.static("public"));
app.use(express.json());

const YOUR_DOMAIN = "http://localhost:4242";

const storeItems = new Map([
  [1, { priceId: "price_XXXXX", name: "Black Sweatsuit" }],
  [2, { priceId: "price_XXXXX", name: "Grey Sweatsuit" }],
  [3, { priceId: "price_XXXXX", name: "Red Sweatsuit" }],
  [4, { priceId: "price_XXXXX", name: "Blue Sweatsuit" }],
  [5, { priceId: "price_XXXXX", name: "Black T-Shirt" }],
  [6, { priceId: "price_XXXXX", name: "Blue T-Shirt" }],
  [7, { priceId: "price_XXXXX", name: "Purple T-Shirt" }],
]);

app.post("/create-checkout-session", async (req, res) => {
  const session = await stripe.checkout.sessions.create({
    line_item: req.body.items.map((item) => {
      const storeItem = storeItems.get(item.id);
      return {
        price_data: {
          currency: "usd",
          product_data: {
            name: storeItem.name,
          },
          unit_amount: storeItem.priceId,
        },
      };
    }),
    mode: "payment",
    success_url: `${YOUR_DOMAIN}/success.html`,
    cancel_url: `${YOUR_DOMAIN}/cancel.html`,
    automatic_tax: { enabled: true },
  });

  res.redirect(303, session.url);
});

app.listen(4242, () => console.log("Running on port 4242"));

checkout.js

const button = document.getElementById("checkout-button");
button.addEventListener("click", () => {
  fetch("/create-checkout-session", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      items: [
        // { id: 1, quantity: 1 },
        // { id: 2, quantity: 1 },
        // { id: 3, quantity: 1 },
        // { id: 4, quantity: 1 },
        // { id: 5, quantity: 1 },
        // { id: 6, quantity: 1 },
        // { id: 7, quantity: 1 },
      ],
    }),
  })
    .then((res) => {
      if (res.ok) return res.json();
      return res.json().then((json) => Promise.reject(json));
    })
    .then(({ url }) => {
      window.location = url;
    })
    .catch((e) => {
      console.error(e.error);
    });
});