Demo Copilot Chat

In this lesson we’ll explore GitHub Copilot Chat — the core of the extension and application — and how it helps you scaffold, iterate, and debug a small Node.js REST API that uses an in-memory database. Goal: build a simple web application in Node.js that exposes a REST API backed by an in-memory database. Each user object will contain an id, firstName, lastName, and email. Start by asking Copilot Chat to scaffold a new project (for example, use /new and request a “Node application API”). Copilot often generates files such as index.js, app.js, and basic tests to get you started.

What Copilot typically produces

index.js — starts the HTTP server.
app.js — defines the Express application and routes.
optional test files — e.g., using Jest + Supertest.

A simple Express application (app.js)

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

app.use(express.json());

// In-memory database
let users = [];

// Create a new user
app.post('/users', (req, res) => {
  const { id, firstName, lastName, email } = req.body;
  const newUser = { id, firstName, lastName, email };
  users.push(newUser);
  res.status(201).json(newUser);
});

// Get all users
app.get('/users', (req, res) => {
  res.json(users);
});

// Get a user by ID
app.get('/users/:id', (req, res) => {
  const user = users.find(u => u.id === Number(req.params.id));
  if (user) {
    res.json(user);
  } else {
    res.status(404).send('User not found');
  }
});

// Update a user by ID
app.put('/users/:id', (req, res) => {
  const id = Number(req.params.id);
  const { firstName, lastName, email } = req.body;
  const user = users.find(u => u.id === id);
  if (user) {
    user.firstName = firstName ?? user.firstName;
    user.lastName = lastName ?? user.lastName;
    user.email = email ?? user.email;
    res.json(user);
  } else {
    res.status(404).send('User not found');
  }
});

// Delete a user by ID
app.delete('/users/:id', (req, res) => {
  const id = Number(req.params.id);
  const index = users.findIndex(u => u.id === id);
  if (index !== -1) {
    users.splice(index, 1);
    res.status(204).send();
  } else {
    res.status(404).send('User not found');
  }
});

module.exports = app;

Server entrypoint (index.js)

const app = require('./app');
const port = process.env.PORT || 3000;

app.listen(port, () => {
  console.log(`Server is running on http://localhost:${port}`);
});

Automated feature tests (app.test.js)

const request = require('supertest');
const app = require('./app');

describe('User API', () => {
  it('should create a new user', async () => {
    const response = await request(app)
      .post('/users')
      .send({ id: 1, firstName: 'John', lastName: 'Doe', email: 'john.doe@example.com' });
    expect(response.statusCode).toBe(201);
    expect(response.body).toHaveProperty('id', 1);
  });

  it('should get all users', async () => {
    const response = await request(app).get('/users');
    expect(response.statusCode).toBe(200);
    expect(Array.isArray(response.body)).toBe(true);
  });

  it('should get a user by ID', async () => {
    const response = await request(app).get('/users/1');
    expect(response.statusCode).toBe(200);
    expect(response.body).toHaveProperty('id', 1);
  });

  it('should update a user by ID', async () => {
    const response = await request(app)
      .put('/users/1')
      .send({ firstName: 'Jane' });
    expect(response.statusCode).toBe(200);
    expect(response.body).toHaveProperty('firstName', 'Jane');
  });

  it('should delete a user by ID', async () => {
    const response = await request(app).delete('/users/1');
    expect(response.statusCode).toBe(204);
  });
});

Minimal package.json

{
  "name": "weatherdemo",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "test": "jest"
  },
  "dependencies": {
    "express": "^4.17.1"
  },
  "devDependencies": {
    "jest": "^27.0.6",
    "supertest": "^6.1.3"
  }
}

API endpoints (quick reference)

Method	Path	Description	Example payload / response
POST	`/users`	Create a new user	Request body: `{"id":1,"firstName":"John","lastName":"Doe","email":"john.doe@example.com"}`
GET	`/users`	List all users	Response: array of user objects
GET	`/users/:id`	Get a single user by id	Response: a user object or 404
PUT	`/users/:id`	Update a user by id	Request body with fields to update
DELETE	`/users/:id`	Remove a user by id	204 on success, 404 if not found

CLI flow (typical)

Install dependencies:

npm install

Expected abbreviated result:

added 408 packages, and audited 409 packages in 8s
48 packages are looking for funding
found 0 vulnerabilities

Start the application:

npm start

Expected output:

> weatherdemo@1.0.0 start
> node index.js

Server is running on http://localhost:3000

Troubleshooting common errors

EJSONPARSE from npm (malformed package.json):

npm ERR! code EJSONPARSE
npm ERR! path /Users/jeremy/Projects/weatherdemo/package.json
npm ERR! JSON.parse Unexpected end of JSON input while parsing empty string

This indicates your package.json is invalid JSON. Paste the contents into Copilot Chat and ask /fix or validate the JSON before running npm install. How Copilot Chat helps in practice

Scaffold project files (/new), generate routes, tests, and README.
Scope fixes with /fix for a file, selection, or working set.
Use /terminal last command to share terminal output with the chat so Copilot can analyze errors and propose patches.

Working set / context matters. Copilot Chat uses your project files (open editors, the full codebase, selected files, and terminal output) to provide focused suggestions. Narrowing the context — a specific file, selection, or terminal output — yields more relevant edits. This behavior is important when using Copilot Chat effectively.

Slash commands and context helpers

/new — scaffold a project or file.
/fix — ask Copilot Chat to fix the selected code or the current working set.
/clear — clear the conversation.
/help — get command usage and interaction info.
/terminal last command — send the last terminal command and output into Copilot Chat.

Hashtags to control context

#changes — include change history context
#codebase — include the full codebase
#editor — include currently open editors
#file — include a specific file
#selection — include just the current selection
#terminalLastCommand and #terminalSelection — include terminal details

Examples to send into the chat:

#file index.js
#selection
/fix

To ask about the last terminal command:

/terminal last command

Practical tips

Accept Copilot edits (e.g., Accept All Edits) to write files into your workspace quickly.
If you change the API shape (for example, remove lastName), update tests, docs, and other references; Copilot can locate inconsistencies when you provide the working set and run /fix.
If Copilot’s response isn’t what you expected, switch models in the UI (e.g., GPT-4o, Claude, Gemini) to compare outputs.

Want more? I can:

produce full starter files packaged in a zip,
add a README and detailed API documentation,
walk through a live debugging session demonstrating /terminal last command and /fix.

Introduction

GitHub Copilot Basics

Prompt Engineering with Copilot

Advanced Features

Management of GitHub Copilot

Using Copilot Efficiently

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