Build SMS OTP Verification with Sinch, Node.js & Express (2FA Guide)

<!-- DEPTH: Title mentions "Next.js & Supabase" but content only covers Express - Critical mismatch (Priority: High) --> <!-- GAP: Missing Next.js and Supabase implementation sections mentioned in title (Type: Critical) -->

SMS OTP verification is essential for securing user authentication in modern web applications. This comprehensive tutorial shows you how to implement two-factor authentication (2FA) using the Sinch Verification API with Node.js and Express. You'll learn to build a production-ready SMS verification system with proper security controls, rate limiting, and database integration.

By following this guide, you'll create an Express application that sends SMS one-time passwords to users' phones and securely verifies their identity—perfect for login authentication, password resets, and transaction confirmations.

Goals:

• Set up a Node.js Express project for OTP verification.
• Integrate the Sinch Verification API for sending and verifying SMS OTPs using direct API calls.
• Store user data (phone number, verification status) potentially using a database (PostgreSQL with Sequelize shown as an example).
• Implement API endpoints for initiating and verifying OTPs.
• Incorporate essential security practices like rate limiting and secure secret management.
• Provide guidance on error handling, logging, deployment, and testing.

<!-- GAP: Missing concrete examples of OTP flow use cases (Type: Substantive) --> <!-- EXPAND: Could include architecture diagram showing client-server-Sinch flow (Type: Enhancement) -->

Technologies Used:

• Node.js: JavaScript runtime environment (LTS versions 20.x or 22.x recommended as of 2025).
• Express: Minimalist web framework for Node.js (v5.1.0+ as of 2025).
• Sinch Verification API: Service for sending and verifying OTPs via SMS, voice, etc. (We'll focus on SMS).
• axios: Promise-based HTTP client for making API calls to Sinch (v1.12.x as of 2025).
• dotenv: Module to load environment variables from a .env file (v16.x).
• (Optional) PostgreSQL & Sequelize: Database and ORM for user data persistence (Sequelize v6.x or v7.x).
• (Optional) express-rate-limit: Middleware for basic brute-force protection (v8.1.x as of 2025).

<!-- GAP: Missing technology comparison or justification for choices (Type: Enhancement) -->

Prerequisites:

• Node.js and npm (or yarn) installed (Node.js 20.x LTS or 22.x LTS recommended).
• A Sinch account (https://dashboard.sinch.com/signup).
• Basic understanding of JavaScript, Node.js, Express, and REST APIs.
• (Optional) PostgreSQL installed and running if you choose the database persistence approach.
• (Optional) A code editor like VS Code.
• (Optional) API testing tool like Postman or curl.

<!-- GAP: Missing link to Sinch pricing information (Type: Substantive) --> <!-- GAP: No mention of Sinch account setup steps (Type: Substantive) -->

Flow:

1. User enters phone number and requests OTP on Frontend.
2. Frontend sends phone number to Backend API (/request-otp).
3. Backend uses an HTTP client (axios) to call the Sinch Verification API, requesting an OTP for the user's phone number.
4. Sinch API sends the OTP code via SMS to the User's Phone.
5. User receives SMS, enters OTP into Frontend.
6. Frontend sends phone number and OTP code to Backend API (/verify-otp). Backend uses the HTTP client to call the Sinch Verification API to verify the code. Optional: Backend updates user verification status in Database. Backend sends success/failure response to Frontend.

<!-- EXPAND: Flow description could be enhanced with sequence diagram (Type: Enhancement) -->

Final Outcome:

By the end of this guide, you will have a functional Node.js Express application capable of sending SMS OTPs via Sinch and verifying them using direct API calls, forming the basis of a secure 2FA system.

<!-- GAP: No mention of what's NOT covered (limitations/scope) (Type: Substantive) -->

Setting Up Your Node.js OTP Project

Let's start by creating our project directory and initializing it with npm.

Step 1: Create Project Directory

Open your terminal and create a new directory for the project, then navigate into it:

mkdir node-sinch-otp
cd node-sinch-otp

Step 2: Initialize npm

Initialize the project using npm. The -y flag accepts default settings.

npm init -y

This creates a package.json file.

Step 3: Install Dependencies

We need Express for our server, dotenv to manage environment variables, and axios to interact with the Sinch REST API.

# Core dependencies
npm install express dotenv axios

# Optional: Database (Sequelize for Postgres)
npm install pg sequelize sequelize-cli

# Optional: Security
npm install express-rate-limit

# Optional: Development dependency
npm install --save-dev nodemon jest supertest winston

• express: Web framework.
• dotenv: Loads environment variables from .env.
• axios: HTTP client to make requests to the Sinch API.
• pg: PostgreSQL client for Node.js (used by Sequelize).
• sequelize: Promise-based Node.js ORM for Postgres, MySQL, etc.
• sequelize-cli: Command-line interface for Sequelize (migrations, seeding).
• express-rate-limit: Basic rate limiting middleware.
• winston: Logger library (used in later steps).
• nodemon: Utility that automatically restarts the server on file changes during development.
• jest, supertest: For unit and integration testing.

<!-- GAP: Missing explanation of when to include optional dependencies (Type: Substantive) --> <!-- GAP: No mention of package version compatibility issues (Type: Substantive) -->

Step 4: Configure nodemon and Test Scripts (Optional)

Open package.json and add/update scripts:

{
  "name": "node-sinch-otp",
  "version": "1.0.0",
  "description": "Node.js Express app for Sinch SMS OTP",
  "main": "src/server.js",
  "scripts": {
    "start": "node src/server.js",
    "start:dev": "nodemon src/server.js",
    "test": "jest",
    "db:migrate": "npx sequelize-cli db:migrate",
    "db:seed:all": "npx sequelize-cli db:seed:all"
  },
  "keywords": [
    "node",
    "express",
    "sinch",
    "otp",
    "2fa",
    "sms"
  ],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "axios": "^1.12.x",
    "dotenv": "^16.x.x",
    "express": "^5.1.x",
    "express-rate-limit": "^8.1.x",
    "pg": "^8.x.x",
    "sequelize": "^6.x.x",
    "sequelize-cli": "^6.x.x",
    "winston": "^3.18.x"
  },
  "devDependencies": {
    "jest": "^29.x.x",
    "nodemon": "^3.x.x",
    "supertest": "^6.x.x"
  }
}

Note: Express 5.1.x requires Node.js 18 or higher. Ensure your Node.js version meets this requirement.

Step 5: Create Project Structure

Organize the project files for better maintainability:

node-sinch-otp/
├── src/
│   ├── controllers/
│   │   └── auth.controller.js
│   ├── services/
│   │   ├── sinch.service.js
│   │   └── user.service.js      # (Optional) DB interactions
│   ├── routes/
│   │   ├── auth.routes.js
│   │   └── index.js             # Main router
│   ├── config/
│   │   ├── index.js             # Centralized config export
│   │   ├── sinch.config.js      # Sinch specific config
│   │   ├── db.config.js         # (Optional) DB config
│   │   ├── database.js          # (Optional) Sequelize-CLI JS config
│   │   └── logger.js            # Logger configuration
│   ├── models/                  # (Optional) Sequelize models
│   │   └── user.model.js
│   ├── migrations/              # (Optional) DB migrations
│   ├── seeders/                 # (Optional) DB seeders
│   ├── utils/                   # Utility functions
│   │   └── response.js
│   └── server.js                # Main Express server setup
├── .env                         # Environment variables (DO NOT COMMIT)
├── .env.example                 # Example environment variables
├── .gitignore                   # Files/folders to ignore in git
├── .sequelizerc                 # (Optional) Sequelize-CLI config file
├── package.json
└── package-lock.json

Create these directories and empty files.

<!-- DEPTH: Structure explanation lacks reasoning for separation of concerns (Priority: Medium) -->

Step 6: Create .gitignore

Create a .gitignore file in the root directory:

# .gitignore

# Dependencies
node_modules/

# Environment variables
.env*
!.env.example

# Logs
logs
*.log
npm-debug.log*
yarn-debug.log*
yarn-error.log*
lerna-debug.log*
error.log
combined.log

# Build output
dist
build

# Operating System Files
.DS_Store
Thumbs.db

# Optional Editor directories and files
.vscode/*
!.vscode/settings.json
!.vscode/tasks.json
!.vscode/launch.json
!.vscode/extensions.json
*.sublime-workspace

# Test results
coverage/

Step 7: Set up Environment Variables (.env)

Create a .env file in the root directory.

# .env
PORT=3000

# Sinch Credentials (Obtain from Sinch Dashboard - Verification API)
# IMPORTANT: Replace these placeholders with your ACTUAL credentials!
# These might be called 'Key ID' & 'Key Secret' or 'Application Key' & 'Application Secret'.
# Verify the exact names and requirements in your Sinch dashboard for the Verification API product.
SINCH_KEY_ID=YOUR_SINCH_KEY_ID
SINCH_KEY_SECRET=YOUR_SINCH_KEY_SECRET
# SINCH_PROJECT_ID=YOUR_SINCH_PROJECT_ID # Might be needed depending on API/auth method used
SINCH_API_BASE_URL=https://verification.api.sinch.com # Check docs for current base URL

# (Optional) Database Credentials
DB_HOST=localhost
DB_PORT=5432
DB_USER=your_db_user
DB_PASSWORD=your_db_password
DB_NAME=your_db_name

# (Optional) Logging Level
LOG_LEVEL=info
NODE_ENV=development

<!-- GAP: Missing detailed instructions for finding credentials in Sinch Dashboard (Type: Substantive) --> <!-- GAP: No explanation of difference between Key ID/Secret vs Application Key/Secret (Type: Substantive) -->

CRITICAL: Replace YOUR_SINCH_KEY_ID and YOUR_SINCH_KEY_SECRET with your actual credentials obtained from the Sinch dashboard. The application will not work without them. Also, update database credentials if you are using the database option.

Never commit your actual .env file to version control. Create a .env.example file with placeholder values to guide other developers.

# .env.example
PORT=3000

SINCH_KEY_ID=REPLACE_WITH_YOUR_SINCH_KEY_ID
SINCH_KEY_SECRET=REPLACE_WITH_YOUR_SINCH_KEY_SECRET
SINCH_API_BASE_URL=https://verification.api.sinch.com

DB_HOST=localhost
DB_PORT=5432
DB_USER=postgres
DB_PASSWORD=password
DB_NAME=sinch_otp_db

LOG_LEVEL=info
NODE_ENV=development

Step 8: Basic Server Setup (src/server.js)

// src/server.js
require('dotenv').config(); // Load .env variables first
const express = require('express');
const mainRouter = require('./routes'); // Assuming main router is in src/routes/index.js
const { errorHandler } = require('./utils/response'); // Basic error handler
const logger = require('./config/logger'); // Import logger
const { connectDB } = require('./config/db.config'); // Optional DB connection

const app = express();
const port = process.env.PORT || 3000;

// Middlewares
app.use(express.json()); // Parse JSON bodies
app.use(express.urlencoded({ extended: true })); // Parse URL-encoded bodies

// API Routes
app.use('/api', mainRouter);

// Health Check endpoint
app.get('/health', (req, res) => {
  res.status(200).json({ status: 'UP', timestamp: new Date().toISOString() });
});

// Global Error Handler Middleware (Should be last)
app.use(errorHandler);

// Start the server
const startServer = async () => {
  try {
    // Only attempt DB connection if DB host is configured
    if (process.env.DB_HOST) {
      await connectDB();
    }
    app.listen(port, () => {
      logger.info(`Server running on http://localhost:${port}`);
    });
  } catch (error) {
    logger.error('Failed to start server:', error);
    process.exit(1);
  }
};

startServer();

module.exports = app; // Export for potential testing

<!-- GAP: Missing CORS configuration for frontend integration (Type: Substantive) --> <!-- GAP: No security headers middleware mentioned (helmet.js) (Type: Substantive) -->

Step 9: Basic Response Utility (src/utils/response.js)

// src/utils/response.js
const logger = require('../config/logger'); // Import logger

/**
 * Sends a standardized success response.
 * @param {object} res - Express response object.
 * @param {number} statusCode - HTTP status code.
 * @param {object|array|string} data - Response payload.
 * @param {string} [message='Success'] - Optional message.
 */
const successResponse = (res, statusCode, data, message = 'Success') => {
  res.status(statusCode).json({
    status: 'success',
    message,
    data,
  });
};

/**
 * Sends a standardized error response.
 * @param {object} res - Express response object.
 * @param {number} statusCode - HTTP status code.
 * @param {string} message - Error message.
 * @param {object} [errorDetails=null] - Optional additional error details (for logging)
 */
const errorResponse = (res, statusCode, message, errorDetails = null) => {
  res.status(statusCode).json({
    status: 'error',
    message,
  });
  // Log the detailed error internally if provided
  if (errorDetails) {
    logger.error(`Responding with ${statusCode}: ${message}`, errorDetails);
  }
};

/**
 * Basic global error handler middleware.
 */
const errorHandler = (err, req, res, next) => {
  logger.error("ERROR:", err.stack || err); // Log the error stack

  // Default to 500 if no status code is set on the error
  const statusCode = err.statusCode || 500;
  const message = err.message || 'Internal Server Error';

  // Avoid sending sensitive error details to the client in production
  const responseMessage = (process.env.NODE_ENV === 'production' && statusCode === 500)
    ? 'Internal Server Error'
    : message;

  res.status(statusCode).json({
    status: 'error',
    message: responseMessage,
  });
};

module.exports = {
  successResponse,
  errorResponse,
  errorHandler,
};

Step 10: Set up Logger (src/config/logger.js)

Create the logger configuration file. Ensure you installed winston in Step 3.

// src/config/logger.js
const winston = require('winston');

const logger = winston.createLogger({
  level: process.env.LOG_LEVEL || 'info',
  format: winston.format.combine(
    winston.format.timestamp(),
    winston.format.errors({ stack: true }), // Log stack traces
    winston.format.splat(),
    winston.format.json() // Log in JSON format
  ),
  defaultMeta: { service: 'node-sinch-otp' },
  transports: [
    // Write all logs with level `error` and below to `error.log`
    new winston.transports.File({ filename: 'error.log', level: 'error' }),
    // Write all logs with level `info` and below to `combined.log`
    new winston.transports.File({ filename: 'combined.log' }),
  ],
});

// If we're not in production then log to the `console` with the format:
// `${info.level}: ${info.message} JSON.stringify({ ...rest }) `
if (process.env.NODE_ENV !== 'production') {
  logger.add(new winston.transports.Console({
    format: winston.format.combine(
      winston.format.colorize(), // Add colors
      winston.format.simple() // Simple format: level: message
    )
  }));
}

module.exports = logger;

<!-- GAP: Missing log rotation configuration for production (Type: Substantive) --> <!-- GAP: No mention of PII/sensitive data filtering in logs (Type: Critical) -->

You should now have a runnable basic Express server structure.

Implementing Sinch SMS OTP Service Layer

Now we'll implement the core SMS OTP functionality by integrating the Sinch Verification API using axios for HTTP requests. This service layer handles sending OTP codes via SMS and verifying user-submitted codes.

Step 1: Configure Sinch API Authentication

First, create a secure configuration file to load your Sinch Verification API credentials from environment variables. This setup uses Basic Authentication for API requests.

// src/config/sinch.config.js
require('dotenv').config();

// Validate essential Sinch environment variables
const requiredSinchVars = ['SINCH_KEY_ID', 'SINCH_KEY_SECRET', 'SINCH_API_BASE_URL'];
requiredSinchVars.forEach(key => {
  if (!process.env[key]) {
    console.error(`FATAL ERROR: Missing required Sinch environment variable: ${key}. Check your .env file.`);
    process.exit(1); // Exit if essential config is missing
  }
});

const sinchConfig = {
  keyId: process.env.SINCH_KEY_ID, // Often used as username in Basic Auth
  keySecret: process.env.SINCH_KEY_SECRET, // Often used as password in Basic Auth
  apiBaseUrl: process.env.SINCH_API_BASE_URL,
  // Add other Sinch config if needed
};

module.exports = sinchConfig;

Next, create the service file to encapsulate Sinch API interactions using axios.

// src/services/sinch.service.js
const axios = require('axios');
const { keyId, keySecret, apiBaseUrl } = require('../config/sinch.config');
const logger = require('../config/logger');

// Create an axios instance for Sinch API calls
const sinchApiClient = axios.create({
  baseURL: `${apiBaseUrl}/verification/v1`, // Assuming v1 verification endpoint base
  auth: {
    username: keyId, // Using Key ID as username for Basic Auth
    password: keySecret // Using Key Secret as password for Basic Auth
  },
  headers: {
    'Content-Type': 'application/json',
    'Accept': 'application/json'
  }
});

<!-- GAP: Missing axios timeout configuration (Type: Substantive) -->
<!-- GAP: No axios retry logic for network failures (Type: Substantive) -->

/**
 * Initiates an SMS OTP verification request via Sinch API.
 * @param {string} phoneNumber - User's phone number in E.164 format (e.g., +1xxxxxxxxxx).
 * @returns {Promise<object&gt;} - Promise resolving with the verification result details (e.g., { id: verificationId })
 * @throws {Error} - Throws error with details if the Sinch API call fails.
 */
const requestOtp = async (phoneNumber) =&gt; {
  logger.info(`Requesting OTP for ${phoneNumber} via Sinch API`);
  const payload = {
    identity: { type: 'number', endpoint: phoneNumber },
    method: 'sms'
    // Add other options like 'reference', 'custom', 'acceptedLanguages' as needed per Sinch docs
    // reference: 'your_internal_reference_id',
    // custom: 'Your custom message identifier or text'
  };

&lt;!-- GAP: Missing phone number format validation before API call (Type: Substantive) --&gt;
&lt;!-- EXPAND: Could include example of custom SMS message template (Type: Enhancement) --&gt;

  try {
    const response = await sinchApiClient.post('/verifications', payload);
    logger.info(`Sinch verification started successfully for ${phoneNumber}`, response.data);
    // Return relevant info, typically the verification ID
    return { id: response.data.id }; // Adjust based on actual Sinch API response structure
  } catch (error) {
    logger.error('Sinch API Error - Request OTP:', {
        message: error.message,
        status: error.response?.status,
        data: error.response?.data,
        config: error.config // Log request details for debugging
    });
    // Provide a more specific error message
    const errorMessage = `Failed to initiate OTP verification with Sinch. Status: ${error.response?.status || 'N/A'}. Reason: ${error.response?.data?.error?.message || error.message}`;
    const serviceError = new Error(errorMessage);
    serviceError.statusCode = error.response?.status || 500; // Pass status code up if available
    serviceError.details = error.response?.data; // Attach details
    throw serviceError;
  }
};

/**
 * Verifies an SMS OTP code via Sinch API using the Verification ID method.
 * @param {string} verificationId - The ID received from the requestOtp call.
 * @param {string} code - The OTP code entered by the user.
 * @returns {Promise<boolean&gt;} - Promise resolving with true if verification is successful, false otherwise.
 * @throws {Error} - Throws error if the Sinch API call fails unexpectedly (e.g., network error, server error).
 */
const verifyOtpById = async (verificationId, code) =&gt; {
  logger.info(`Verifying OTP for verification ID ${verificationId} with code ${code}`);
  const payload = {
    method: 'sms',
    sms: { code: code }
  };

&lt;!-- GAP: Missing code format validation (length, numeric) (Type: Substantive) --&gt;

  try {
    // Sinch uses PUT to report verification result against the ID
    const response = await sinchApiClient.put(`/verifications/id/${verificationId}`, payload);
    logger.info(`Sinch verification report result for ID ${verificationId}:`, response.data);

    // Check the actual success status field from the API response
    // Common success status is 'SUCCESSFUL', verify this in Sinch docs
    return response.data.status === 'SUCCESSFUL';

  } catch (error) {
    logger.error(`Sinch API Error - Verify OTP by ID ${verificationId}:`, {
        message: error.message,
        status: error.response?.status,
        data: error.response?.data,
    });

    // Specific handling for common failure cases (like invalid code)
    // Check if Sinch returns a specific status code or error message for invalid OTP
    // Example: Assuming 4xx status indicates client error (like wrong code) vs 5xx server error
    if (error.response && error.response.status &gt;= 400 && error.response.status &lt; 500) {
        // Common reasons: Invalid code_ verification expired_ verification already used_ verification not found
        logger.warn(`Verification failed for ID ${verificationId}. Status: ${error.response.status}. Reason: ${error.response?.data?.error?.message || 'Client error'}`);
        return false; // Treat client-side errors (like wrong code) as verification failure
    }

    // For other errors (network_ 5xx)_ re-throw a service error
    const errorMessage = `Failed to verify OTP code with Sinch for ID ${verificationId}. Status: ${error.response?.status || 'N/A'}. Reason: ${error.response?.data?.error?.message || error.message}`;
    const serviceError = new Error(errorMessage);
    serviceError.statusCode = error.response?.status || 500;
    serviceError.details = error.response?.data;
    throw serviceError;
  }
};


/**
 * Verifies an SMS OTP code via Sinch API using the Phone Number method.
 * NOTE: This might require a different endpoint or payload structure depending on the Sinch API version.
 *       Using the verification ID method (`verifyOtpById`) is generally preferred if the ID is available.
 * @param {string} phoneNumber - User's phone number in E.164 format.
 * @param {string} code - The OTP code entered by the user.
 * @returns {Promise<boolean&gt;} - Promise resolving with true if verification is successful, false otherwise.
 * @throws {Error} - Throws error if the Sinch API call fails unexpectedly.
 */
const verifyOtpByNumber = async (phoneNumber, code) =&gt; {
    logger.info(`Verifying OTP for number ${phoneNumber} with code ${code}`);
    const payload = {
        method: 'sms',
        sms: { code: code }
    };

    try {
        // Sinch often uses PUT to report verification result against the number
        const response = await sinchApiClient.put(`/verifications/number/${phoneNumber}`, payload);
        logger.info(`Sinch verification report result for number ${phoneNumber}:`, response.data);

        // Check the actual success status field from the API response
        return response.data.status === 'SUCCESSFUL';

    } catch (error) {
        logger.error(`Sinch API Error - Verify OTP by Number ${phoneNumber}:`, {
            message: error.message,
            status: error.response?.status,
            data: error.response?.data,
        });

        // Specific handling for common failure cases (like invalid code)
        if (error.response && error.response.status &gt;= 400 && error.response.status &lt; 500) {
            logger.warn(`Verification failed for number ${phoneNumber}. Status: ${error.response.status}. Reason: ${error.response?.data?.error?.message || 'Client error'}`);
            return false; // Treat client-side errors (like wrong code) as verification failure
        }

        // For other errors (network_ 5xx)_ re-throw a service error
        const errorMessage = `Failed to verify OTP code with Sinch for number ${phoneNumber}. Status: ${error.response?.status || 'N/A'}. Reason: ${error.response?.data?.error?.message || error.message}`;
        const serviceError = new Error(errorMessage);
        serviceError.statusCode = error.response?.status || 500;
        serviceError.details = error.response?.data;
        throw serviceError;
    }
};


module.exports = {
  requestOtp_
  // Choose one verification method based on your flow and Sinch API docs:
  verifyOtp: verifyOtpByNumber_ // Or verifyOtpById if you store/pass the verification ID
  // verifyOtp: verifyOtpById
};

<!-- DEPTH: Verification methods need clearer guidance on when to use which (Priority: Medium) --> <!-- GAP: Missing rate limiting on service layer (Type: Substantive) -->

Step 2: Set Up PostgreSQL Database for User Verification Status

For production applications_ you'll want to persist user verification status. We'll use PostgreSQL with Sequelize ORM to store phone numbers and track which users have completed SMS verification.

• (a) Configure Sequelize CLI: Initialize Sequelize if you haven't:

  bash

  npx sequelize-cli init
  

  This creates config/config.json_ models/index.js_ etc. We want sequelize-cli to use our .env variables. Create a .sequelizerc file in the project root:

  javascript

  // .sequelizerc
  const path = require('path');
  
  module.exports = {
    'config': path.resolve('src'_ 'config'_ 'database.js')_ // Path to JS config file
    'models-path': path.resolve('src'_ 'models')_
    'seeders-path': path.resolve('src'_ 'seeders')_
    'migrations-path': path.resolve('src'_ 'migrations')
  };
  

  Now_ create the JS configuration file referenced above (src/config/database.js):

  javascript

  // src/config/database.js
  require('dotenv').config(); // Load .env variables
  
  // Define config for different environments
  // Reads from process.env_ ensuring sensitive creds aren't hardcoded
  module.exports = {
    development: {
      username: process.env.DB_USER_
      password: process.env.DB_PASSWORD_
      database: process.env.DB_NAME_
      host: process.env.DB_HOST_
      port: process.env.DB_PORT || 5432_
      dialect: 'postgres'_ // Or your chosen dialect
      dialectOptions: { // Example: SSL for production Postgres
        // ssl: { require: true_ rejectUnauthorized: false } // Adjust as needed
      }
    }_
    test: {
      // Configuration for test environment (e.g._ separate test DB)
      username: process.env.DB_USER_TEST || process.env.DB_USER_
      password: process.env.DB_PASSWORD_TEST || process.env.DB_PASSWORD_
      database: process.env.DB_NAME_TEST || `${process.env.DB_NAME}_test`_
      host: process.env.DB_HOST_TEST || process.env.DB_HOST_
      port: process.env.DB_PORT_TEST || 5432_
      dialect: 'postgres'_
      logging: false_ // Disable logging for tests
    }_
    production: {
      // Configuration for production environment
      username: process.env.DB_USER_PROD || process.env.DB_USER_
      password: process.env.DB_PASSWORD_PROD || process.env.DB_PASSWORD_
      database: process.env.DB_NAME_PROD || process.env.DB_NAME_
      host: process.env.DB_HOST_PROD || process.env.DB_HOST_
      port: process.env.DB_PORT_PROD || 5432_
      dialect: 'postgres'_
      logging: false_ // Usually disable verbose logging in prod
      dialectOptions: {
        // ssl: { require: true_ rejectUnauthorized: false } // Example for cloud DBs
      }_
      pool: { // Production pool settings
        max: 10_
        min: 1_
        acquire: 30000_
        idle: 10000
      }
    }
  };
  

<!-- GAP: Missing SSL certificate configuration details (Type: Substantive) -->

• (b) Configure Database Connection (src/config/db.config.js): This file sets up the Sequelize instance for the application runtime.

  javascript

  // src/config/db.config.js
  require('dotenv').config();
  const { Sequelize } = require('sequelize');
  const dbCliConfig = require('./database'); // Load the CLI config
  const logger = require('./logger');
  
  // Determine the environment (default to development)
  const env = process.env.NODE_ENV || 'development';
  const config = dbCliConfig[env]; // Get the config for the current environment
  
  let sequelizeInstance = null;
  let connectDBFunction = async () => logger.info('DB connection skipped (config incomplete).');
  
  if (config && config.database && config.username && config.host) {
      sequelizeInstance = new Sequelize(config.database, config.username, config.password, {
        host: config.host,
        port: config.port,
        dialect: config.dialect,
        logging: (msg) => logger.debug(msg), // Log SQL via logger debug level
        pool: config.pool || { max: 5, min: 0, acquire: 30000, idle: 10000 },
        dialectOptions: config.dialectOptions || {},
      });
  
      connectDBFunction = async () => {
        try {
          await sequelizeInstance.authenticate();
          logger.info('Database connection established successfully.');
          // DO NOT use sync() in production. Use migrations.
          // await sequelizeInstance.sync({ alter: process.env.NODE_ENV === 'development' });
        } catch (error) {
          logger.error('Unable to connect to the database:', error);
          throw error; // Re-throw error to be caught by server start
        }
      };
  } else {
      logger.warn('Database configuration is incomplete. Skipping DB connection setup.');
  }
  
  module.exports = { sequelize: sequelizeInstance, connectDB: connectDBFunction };
  

• (c) Update server.js to connect DB: This was already handled in the src/server.js code provided in Section 1, Step 8. It checks process.env.DB_HOST before calling connectDB.

• (d) Create User Model (src/models/user.model.js):

  javascript

  // src/models/user.model.js
  const { DataTypes } = require('sequelize');
  const { sequelize } = require('../config/db.config'); // Use the configured instance
  const logger = require('../config/logger');
  
  // Check if sequelize instance exists (it might be null if DB config is missing)
  if (!sequelize) {
    logger.warn('Sequelize instance is null. Skipping User model definition.');
    module.exports = null; // Export null if DB is not configured
  } else {
      const User = sequelize.define('User', {
        id: {
          type: DataTypes.UUID,
          defaultValue: DataTypes.UUIDV4,
          primaryKey: true,
        },
        phoneNumber: {
          type: DataTypes.STRING,
          allowNull: false,
          unique: true,
          validate: {
            // Basic E.164 format check. Consider using google-libphonenumber for robust validation.
            isE164(value) {
               if(!/^\+[1-9]\d{1,14}$/.test(value)) {
                  throw new Error('Phone number must be in E.164 format (e.g., +1xxxxxxxxxx)');
               }
            }
          }
        },
        isVerified: {
          type: DataTypes.BOOLEAN,
          allowNull: false,
          defaultValue: false,
        },
        // Add other fields as needed: name, email, verificationId (optional) etc.
        // verificationId: { // Optionally store the latest Sinch verification ID
        //   type: DataTypes.STRING,
        //   allowNull: true,
        // },
      }, {
        timestamps: true, // Automatically adds createdAt and updatedAt
        tableName: 'Users', // Explicitly define table name
      });
  

<!-- GAP: Missing index strategy discussion (Type: Substantive) --> <!-- EXPAND: Could add soft delete functionality (Type: Enhancement) -->

module.exports = User; }

*   **(e) Create Migration File:**
Use Sequelize CLI. It will now use `src/config/database.js` because of `.sequelizerc`.
```bash
npx sequelize-cli model:generate --name User --attributes phoneNumber:string,isVerified:boolean

Then, edit the generated migration file in src/migrations/ to match the model precisely (UUID primary key, constraints, timestamps, index). The generated file will have a timestamp in its name (e.g., YYYYMMDDHHMMSS-create-user.js).

// src/migrations/YYYYMMDDHHMMSS-create-user.js
'use strict';
/** @type {import('sequelize-cli').Migration} */
module.exports = {
  async up(queryInterface, Sequelize) {
    await queryInterface.createTable('Users', {
      id: {
        allowNull: false,
        primaryKey: true,
        type: Sequelize.UUID,
        defaultValue: Sequelize.UUIDV4,
      },
      phoneNumber: {
        type: Sequelize.STRING,
        allowNull: false,
        unique: true,
      },
      isVerified: {
        type: Sequelize.BOOLEAN,
        allowNull: false,
        defaultValue: false,
      },
      // verificationId: { // Add if you included in model
      //   type: Sequelize.STRING,
      //   allowNull: true,
      // },
      createdAt: {
        allowNull: false,
        type: Sequelize.DATE,
      },
      updatedAt: {
        allowNull: false,
        type: Sequelize.DATE,
      },
    });
    // Add an index on phoneNumber for faster lookups
    await queryInterface.addIndex('Users', ['phoneNumber']);
  },
  async down(queryInterface, Sequelize) {
    await queryInterface.dropTable('Users');
  },
};

• (f) Run Migrations: The script added to package.json earlier (db:migrate) will work. Ensure your database exists and credentials in .env are correct for the NODE_ENV environment (defaults to development).
  bash

  npm run db:migrate
  
  To run migrations for production: NODE_ENV=production npm run db:migrate

<!-- GAP: Missing migration rollback instructions (Type: Substantive) --> <!-- GAP: No guidance on handling migration failures (Type: Substantive) -->

• (g) Create User Service (src/services/user.service.js):
  javascript

  // src/services/user.service.js
  const User = require('../models/user.model'); // Might be null if DB not configured
  const logger = require('../config/logger');
  
  /**
   * Finds or creates a user by phone number.
   * @param {string} phoneNumber - User's phone number in E.164 format.
   * @returns {Promise<User|null&gt;} - The found or newly created user instance, or null if DB is not configured.
   * @throws {Error} - If a database operation fails.
   */
  const findOrCreateUser = async (phoneNumber) =&gt; {
    if (!User) {
      logger.warn('User model not available, skipping findOrCreateUser.');
      return null; // Skip if DB model is not loaded
    }
  
    try {
      const [user, created] = await User.findOrCreate({
        where: { phoneNumber: phoneNumber },
        defaults: {
          phoneNumber: phoneNumber,
          isVerified: false,
        },
      });
      if (created) {
        logger.info(`New user created for ${phoneNumber}`);
      } else {
        logger.info(`Existing user found for ${phoneNumber}`);
      }
      return user;
    } catch (error) {
      logger.error(`Error finding or creating user for ${phoneNumber}:`, error);
      // Re-throw a more specific error or handle as needed
      throw new Error(`Database error while processing user ${phoneNumber}.`);
    }
  };
  
  /**
   * Updates a user's verification status.
   * @param {string} phoneNumber - The phone number of the user to update.
   * @param {boolean} isVerified - The new verification status.
   * @returns {Promise<boolean&gt;} - True if the update was successful (at least one row affected), false otherwise or if DB not configured.
   * @throws {Error} - If a database operation fails.
   */
  const updateUserVerificationStatus = async (phoneNumber, isVerified) =&gt; {
      if (!User) {
          logger.warn('User model not available, skipping updateUserVerificationStatus.');
          return false;
      }
  
      try {
          const [affectedRows] = await User.update(
              { isVerified: isVerified },
              { where: { phoneNumber: phoneNumber } }
          );
          if (affectedRows &gt; 0) {
              logger.info(`Updated verification status for ${phoneNumber} to ${isVerified}`);
              return true;
          } else {
              logger.warn(`No user found with phone number ${phoneNumber} to update verification status.`);
              return false;
          }
      } catch (error) {
          logger.error(`Error updating verification status for ${phoneNumber}:`, error);
          throw new Error(`Database error while updating verification status for ${phoneNumber}.`);
      }
  };
  
  
  module.exports = {
    findOrCreateUser,
    updateUserVerificationStatus,
    // Add other user-related functions here (e.g., findUserByPhoneNumber)
  };
  

<!-- GAP: Missing transaction handling for data consistency (Type: Substantive) --> <!-- GAP: No mention of connection pooling best practices (Type: Enhancement) -->

Frequently Asked Questions About SMS OTP Implementation

<!-- DEPTH: FAQ section provides good breadth but lacks implementation details (Priority: Medium) --> <!-- GAP: Missing controller and routes implementation sections before FAQ (Type: Critical) --> <!-- GAP: Missing testing section (Type: Critical) --> <!-- GAP: Missing deployment guidance (Type: Critical) --> <!-- GAP: Missing monitoring and observability section (Type: Substantive) -->

What is the Sinch Verification API and how does SMS OTP work?

The Sinch Verification API is a service that enables SMS-based phone number verification through One-Time Passwords (OTP). It works by sending a unique code via SMS to a user's phone number, which they must enter to verify their identity. The API handles the SMS delivery, code generation, and verification logic, supporting multiple methods including SMS, voice calls, and flash calls. You integrate it via REST API calls using Basic Authentication (for testing) or Application Signed Requests (for production).

<!-- EXPAND: Could include cost breakdown or API limits (Type: Enhancement) -->

Which Node.js version should I use for SMS OTP in 2025?

Use Node.js 20.x LTS or Node.js 22.x LTS for production applications as of 2025. Node.js 20 "Iron" receives LTS support until April 2026, while Node.js 22 "Jod" entered Active LTS in October 2024 with support until April 2027. Express 5.1.x (the latest version) requires Node.js 18 or higher, making these LTS versions the recommended choice for new projects.

Should I use Basic Auth or Application Signed Requests for Sinch API?

Basic Authentication uses your application key and secret directly in API requests (as HTTP Basic Auth). It's quick to implement and suitable for testing and prototyping. Application Signed Requests provide enhanced security through cryptographic request signing and are recommended for production environments. You can configure the minimum required authentication level in the Sinch Dashboard. For production deployments, always migrate to Application Signed Requests to protect your credentials.

<!-- GAP: Missing code example showing Application Signed Requests implementation (Type: Substantive) -->

How do I prevent SMS OTP abuse with rate limiting?

Use the express-rate-limit middleware (v8.1.x as of 2025) to protect OTP endpoints from brute-force attacks. Configure stricter limits for OTP request endpoints (e.g., 3 requests per 15 minutes per phone number) and verification endpoints (e.g., 5 attempts per 15 minutes). Implement both IP-based and phone number-based rate limiting. Store rate limit data in Redis for distributed systems. Always log rate limit violations for security monitoring.

<!-- GAP: Missing actual rate limiting implementation code (Type: Critical) -->

Should I use Sequelize or another ORM for storing user verification data?

Sequelize (v6.x or v7.x) is a mature, feature-rich ORM that works well for PostgreSQL user data storage. It provides migration support, model validation, and relationship management. Alternatives include Prisma (modern TypeScript-first ORM with excellent DX), TypeORM (good TypeScript support), or Drizzle (lightweight, type-safe). Choose based on your team's expertise and project requirements. For simple OTP verification, you only need to store phone numbers, verification status, and timestamps.

<!-- EXPAND: Could include ORM performance comparison table (Type: Enhancement) -->

How should I handle expired OTP codes and verification failures?

Sinch API returns specific status codes for different failure scenarios. For 4xx client errors (invalid code, expired verification, verification not found), treat these as verification failures and return false to the user with appropriate error messages. For 5xx server errors or network issues, implement retry logic with exponential backoff. Set reasonable OTP expiration times (typically 5-10 minutes) and limit verification attempts (3-5 attempts) before requiring a new OTP request.

<!-- GAP: Missing specific error codes reference table (Type: Substantive) -->

What are the best security practices for implementing SMS 2FA?

Implement these security measures: (1) Use rate limiting on both request and verify endpoints, (2) Store sensitive credentials in environment variables, never in code, (3) Use HTTPS for all API communications, (4) Implement audit logging for all OTP requests and verifications, (5) Set reasonable OTP expiration times, (6) Limit verification attempts per OTP, (7) Use Application Signed Requests in production, (8) Validate phone numbers in E.164 format, (9) Implement GDPR-compliant data handling, and (10) Monitor for suspicious patterns and account takeover attempts.

<!-- EXPAND: Could include security checklist or threat model diagram (Type: Enhancement) -->

How do I migrate from Basic Authentication to Application Signed Requests in production?

Consult the official Sinch documentation at https://developers.sinch.com/docs/verification/api-reference/authentication/ for migration details. Application Signed Requests require generating request signatures using your application key and secret. Most official Sinch SDKs (Node.js, Python, Java, .NET) handle signing automatically. Update your Sinch API client configuration to use signed requests, test thoroughly in a staging environment, then configure the minimum authentication level in the Sinch Dashboard to enforce signed requests before deploying to production.

<!-- GAP: Missing step-by-step migration guide with code examples (Type: Substantive) -->