🔍 Developer Error Code Database

ps aux | grep node  # Check if Node.js server is running
netstat -tuln | grep LISTEN  # Check listening ports

npm start  # or yarn start, npm run dev, etc.

// Node.js
server.timeout = 120000; // 2 minutes

// Axios
axios.get(url, { timeout: 30000 })

// Retry with exponential backoff
const retry = async (fn, retries = 3) => {
  for (let i = 0; i < retries; i++) {
    try {
      return await fn();
    } catch (err) {
      if (i === retries - 1) throw err;
      await new Promise(r => setTimeout(r, 1000 * Math.pow(2, i)));
    }
  }
};

// Fetch API
fetch(url, { signal: AbortSignal.timeout(10000) })

// Axios
axios.get(url, { timeout: 10000 })

ping google.com
traceroute google.com  # or tracert on Windows

nslookup yourdomain.com
dig yourdomain.com

ping hostname
telnet hostname port

# For React
PORT=3001 npm start

# For Node.js/Express
const PORT = process.env.PORT || 3001;

# For Vite
npm run dev -- --port 3001

// Instead of port 80, use 8080
const PORT = 8080;

sudo setcap 'cap_net_bind_service=+ep' $(which node)

npm install --legacy-peer-deps

npm install --force

npm cache clean --force
rm -rf node_modules package-lock.json
npm install

"dependencies": {
  "react": "^18.0.0",  // Make sure compatible versions
  "react-dom": "^18.0.0"
}

npm cache clean --force

rm -rf node_modules package-lock.json
npm install

npm install --registry=https://registry.npmjs.org/

ls -la path/to/file  # Unix
dir path\to\file     # Windows

rm -rf node_modules
npm install

npx create-react-app my-app  # Instead of global install

mkdir ~/.npm-global
npm config set prefix '~/.npm-global'
echo 'export PATH=~/.npm-global/bin:$PATH' >> ~/.profile
source ~/.profile

sudo npm install -g package-name

git pull origin main --allow-unrelated-histories

git status  # See conflicted files
# Edit files to resolve conflicts
git add .
git commit -m "Merge unrelated histories"

git pull origin main
# Resolve any conflicts if needed
git push origin main

git pull --rebase origin main
git push origin main

git push --force origin main  # Overwrites remote!

git status

<<<<<<< HEAD
Your changes
=======
Their changes
>>>>>>> branch-name

git add conflicted-file.js
git commit -m "Resolve merge conflict"

git merge --abort

ssh-keygen -t ed25519 -C "[email protected]"

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

cat ~/.ssh/id_ed25519.pub
# Copy the output and add to GitHub Settings > SSH Keys

ssh -T [email protected]

git remote set-url origin https://github.com/username/repo.git

sudo systemctl start mysql       # MySQL
sudo systemctl start postgresql  # PostgreSQL
sudo systemctl start mongod      # MongoDB

DB_HOST=localhost
DB_USER=your_username
DB_PASSWORD=your_password
DB_NAME=your_database

-- MySQL
GRANT ALL PRIVILEGES ON database_name.* TO 'username'@'localhost';

-- PostgreSQL
GRANT ALL PRIVILEGES ON DATABASE database_name TO username;

// Node.js with MySQL
connection.end();

// Using pool
pool.query(sql, (err, result) => {
  // Don't forget to release
  connection.release();
});

const pool = mysql.createPool({
  host: 'localhost',
  user: 'user',
  password: 'password',
  database: 'db',
  connectionLimit: 10
});

-- MySQL
SET GLOBAL max_connections = 200;

-- PostgreSQL (in postgresql.conf)
max_connections = 200

npm install module-name
# or
yarn add module-name

// Correct relative paths
import Component from './Component';  // Same directory
import Component from '../Component'; // Parent directory

// Node modules
import express from 'express';

rm -rf node_modules package-lock.json
npm install

// Use .jsx or .tsx extension for files with JSX
// Or configure babel to handle .js files with JSX

// Wrong
import App from './App.html';

// Correct
import App from './App.js';

NODE_OPTIONS="--max-old-space-size=4096" npm run build

# Or in package.json
"scripts": {
  "build": "NODE_OPTIONS=--max-old-space-size=4096 react-scripts build"
}

npm cache clean --force
rm -rf node_modules .next build
npm install
npm run build

const cors = require('cors');

// Allow all origins (development only)
app.use(cors());

// Or specific origin (production)
app.use(cors({
  origin: 'http://localhost:3000',
  credentials: true
}));

@Configuration
public class WebConfig {
  @Bean
  public WebMvcConfigurer corsConfigurer() {
    return new WebMvcConfigurer() {
      @Override
      public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/**")
          .allowedOrigins("http://localhost:3000")
          .allowedMethods("GET", "POST", "PUT", "DELETE");
      }
    };
  }
}

// package.json
{
  "proxy": "http://localhost:8080"
}

// Then use relative URLs
fetch('/api/data')

// vite.config.js
export default {
  server: {
    proxy: {
      '/api': 'http://localhost:8080'
    }
  }
}

app.use((req, res, next) => {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Methods', 'GET, POST, PUT, DELETE');
  res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization');

  if (req.method === 'OPTIONS') {
    return res.sendStatus(200);
  }
  next();
});

# Install django-cors-headers
pip install django-cors-headers

# settings.py
INSTALLED_APPS = [
    'corsheaders',
    ...
]

MIDDLEWARE = [
    'corsheaders.middleware.CorsMiddleware',
    ...
]

CORS_ALLOWED_ORIGINS = [
    "http://localhost:3000",
]