Here's a quick guide to building a REST API with Express.js:

1. Setup

npm init -y
npm install express

2. Create your server

const express = require('express');
const app = express();

app.use(express.json());

let items = [
  { id: 1, name: 'Item One' },
  { id: 2, name: 'Item Two' },
];

app.get('/api/items', (req, res) => {
  res.json(items);
});

app.post('/api/items', (req, res) => {
  const item = { id: items.length + 1, ...req.body };
  items.push(item);
  res.status(201).json(item);
});

app.listen(3000, () => console.log('Running on :3000'));

Key principles:

Use proper HTTP methods (GET, POST, PUT, DELETE)

Return appropriate status codes

Validate input data

Handle errors gracefully