# Python
prices = [499, 1299, 250]
expensive = [p for p in prices if p > 400]
print(sum(expensive))

// JavaScript (Node.js)
const prices = [499, 1299, 250];
const expensive = prices.filter(p => p > 400);
console.log(expensive.reduce((a, b) => a + b, 0));

# Python
python -m venv .venv && source .venv/bin/activate
pip install fastapi uvicorn

# Node.js
mkdir api && cd api
npm init -y
npm install express
node --version   # check you have a current LTS release

// Run two independent requests at the same time
async function load() {
  const [users, posts] = await Promise.all([
    fetch("https://jsonplaceholder.typicode.com/users").then(r => r.json()),
    fetch("https://jsonplaceholder.typicode.com/posts").then(r => r.json()),
  ]);
  console.log(users.length, "users,", posts.length, "posts");
}
load().catch(err => console.error("failed:", err.message));
// Node 18+ has fetch built in

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

// BAD: a long CPU loop blocks every other request
app.get("/slow", (req, res) => {
  let total = 0;
  for (let i = 0; i < 5e9; i++) total += i;
  res.json({ total });
});

// Better: move heavy work to a worker thread, a job queue,
// or a separate service (often a Python service is a good fit)

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

const items = new Map([[1, { id: 1, name: "Notebook" }]]);

app.get("/items/:id", (req, res) => {
  const item = items.get(Number(req.params.id));
  if (!item) return res.status(404).json({ error: "not found" });
  res.json(item);
});

app.post("/items", (req, res) => {
  if (!req.body.name) return res.status(400).json({ error: "name required" });
  const id = items.size + 1;
  items.set(id, { id, name: req.body.name });
  res.status(201).json(items.get(id));
});

app.listen(3000, () => console.log("http://localhost:3000"));

from fastapi import FastAPI, HTTPException
from pydantic import BaseModel

app = FastAPI()
items = {1: {"id": 1, "name": "Notebook"}}

class NewItem(BaseModel):
    name: str

@app.get("/items/{item_id}")
def get_item(item_id: int):
    if item_id not in items:
        raise HTTPException(status_code=404, detail="not found")
    return items[item_id]

@app.post("/items", status_code=201)
def add_item(body: NewItem):
    new_id = len(items) + 1
    items[new_id] = {"id": new_id, "name": body.name}
    return items[new_id]

Node.js plan for Python developers
Step 1: Modern JavaScript — let/const, arrow functions, destructuring, modules
Step 2: Promises and async/await; Promise.all
Step 3: npm, package.json, scripts
Step 4: Express routes, middleware, error handling
Step 5: Database access (SQL client or ORM)
Step 6: Testing with a JS test runner
Step 7: Rebuild one of your Python APIs in Node

Python or Node.js for this project?
☐ Heavy data processing or ML → Python
☐ Real-time features (chat, live updates) → Node.js is a strong fit
☐ Team is mostly JavaScript → Node.js
☐ Team is mostly Python → Python
☐ Need both → Node API + Python service
Decision: ______  Reason: ______________

Porting a Python API to Node.js
☐ Routes and status codes match
☐ Input validation added for every POST/PUT
☐ Errors return the same JSON shape
☐ Environment variables for config
☐ Tests for each endpoint
☐ README explains how to run both versions