Developing a full-fledged eCommerce website requires a lot of planning, designing, and coding. It involves creating an architecture, designing the UI, and integrating various technologies such as payment gateways, security features, and more. Here is a high-level overview of the architecture and code for an eCommerce website with JavaScript:

1. Architecture: The architecture of an eCommerce website can be divided into the following layers:

• Presentation Layer: This layer consists of the user interface that interacts with the user.

• Application Layer: This layer contains the business logic that processes the user requests and responds to them.

• Data Access Layer: This layer interacts with the database and performs CRUD operations.

1. UI Design: The UI design is an important aspect of an eCommerce website. It should be intuitive, responsive, and visually appealing. Here are some key features of an eCommerce website UI:

• Search bar for easy product search

• Product categories and filters

• Shopping cart to keep track of selected items

• Secure login and checkout process

• Product images and detailed descriptions

• Product reviews and ratings

1. Front-end Development: To develop the front-end of the eCommerce website, you can use popular JavaScript libraries such as React, Angular, or Vue.js. Here are some key features to implement:

• Product listing and detail pages

• Shopping cart page

• Checkout page with payment gateway integration

• User registration and login page

• User account page to manage orders, address, and payment information

1. Back-end Development: The back-end of an eCommerce website can be developed using Node.js, Express.js, and MongoDB. Here are some key features to implement:

• APIs to fetch and update product information

• APIs to handle user authentication and authorization

• APIs to process user orders and payments

• APIs to handle user account management and address information

1. Security: Security is a critical aspect of an eCommerce website. Here are some security measures to implement:

• HTTPS encryption to secure data transmission

• Implementing a firewall and other security measures to protect against attacks

• PCI-DSS compliance to protect user payment information

• Regular security audits and vulnerability assessments

Here's a sample code snippet for an eCommerce website using Node.js, Express.js, and MongoDB:

const express = require('express');
const bodyParser = require('body-parser');
const mongoose = require('mongoose');
const Product = require('./models/product');

const app = express();

mongoose.connect('mongodb://localhost/ecommerce');

app.use(bodyParser.json());

app.get('/api/products', async (req, res) => {
  const products = await Product.find();
  res.json(products);
});

app.post('/api/products', async (req, res) => {
  const product = new Product({
    name: req.body.name,
    price: req.body.price,
    description: req.body.description,
    imageUrl: req.body.imageUrl
  });
  await product.save();
  res.json(product);
});

app.listen(3000, () => {
  console.log('Server started on port 3000');
});

This code creates an API to fetch and create new products in the MongoDB database. You can build upon this code to develop the complete eCommerce website with JavaScript.