Implementing SMS Authentication in React Native Expo App in 5 Minutes (Zero Paperwork)

When building a mobile app, adding SMS Authentication (OTP) for sign-ups or password recovery is often essential. However, when developers start looking for an SMS API provider, they frequently hit a massive roadblock: the paperwork.

Business registration certificates, proof of telecommunication service, pre-registering caller IDs... For solo developers building side projects or startups rushing to launch an MVP, a 2 to 3-day verification delay is a nightmare.

In this tutorial, we will learn how to implement SMS verification in a React Native (Expo) app in just 5 minutes, with absolutely zero paperwork.

💡 The Solution: A Simple 2-Endpoint Architecture

Using a developer-centric API, you only need two endpoints to completely handle SMS authentication: POST /send and POST /verify.

• Send: Dispatches a 6-digit OTP to the provided phone number.
• Verify: Checks if the code the user entered matches the dispatched OTP.

🛠️ Step-by-Step Implementation

Let's look at how to manage UI states and call the APIs in a React Native environment.

1. Setting Up State Variables

We need state hooks for the phone number, the verification code, and the UI state (whether the code has been sent).

const [phoneNumber, setPhoneNumber] = useState('');
const [verificationCode, setVerificationCode] = useState('');
const [isSent, setIsSent] = useState(false);

2. The Complete Code (App.tsx)

Here is the complete, fully functional code you can copy and paste directly into your Expo project.

import React, { useState } from 'react';
import { StyleSheet, Text, View, TextInput, TouchableOpacity, Alert } from 'react-native';

export default function App() {
  const [phoneNumber, setPhoneNumber] = useState('');
  const [verificationCode, setVerificationCode] = useState('');
  const [isSent, setIsSent] = useState(false);

  // Tip: In production, proxy these calls through your own backend to hide the API Key
  const API_KEY = 'YOUR_EASYAUTH_API_KEY';
  const BASE_URL = 'https://api.easyauth.kr';

  // 1. Function to send OTP
  const handleSendCode = async () => {
    if (phoneNumber.length < 10) {
      Alert.alert('Error', 'Please enter a valid phone number.');
      return;
    }

    try {
      const response = await fetch(`${BASE_URL}/send`, {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          'Authorization': `Bearer ${API_KEY}`
        },
        body: JSON.stringify({ to: phoneNumber })
      });

      if (response.ok) {
        setIsSent(true);
        Alert.alert('Success', 'Verification code sent!');
      } else {
        Alert.alert('Error', 'Failed to send code.');
      }
    } catch (error) {
      console.error(error);
      Alert.alert('Error', 'Network error occurred.');
    }
  };

  // 2. Function to verify OTP
  const handleVerifyCode = async () => {
    if (verificationCode.length !== 6) {
      Alert.alert('Error', 'Please enter the 6-digit code.');
      return;
    }

    try {
      const response = await fetch(`${BASE_URL}/verify`, {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          'Authorization': `Bearer ${API_KEY}`
        },
        body: JSON.stringify({
          to: phoneNumber,
          code: verificationCode
        })
      });

      if (response.ok) {
        Alert.alert('Success', 'Authentication completed!');
        // Navigate to the next screen
      } else {
        Alert.alert('Error', 'Invalid verification code.');
      }
    } catch (error) {
      console.error(error);
      Alert.alert('Error', 'An error occurred during verification.');
    }
  };

  return (
    
      Mobile Verification
      
      
      
      {!isSent ? (
        
          Get Code
        
      ) : (
        <>
          
          
            Verify
          
        </>
      )}
    
  );
}

const styles = StyleSheet.create({
  container: { flex: 1, padding: 20, justifyContent: 'center', backgroundColor: '#fff' },
  title: { fontSize: 24, fontWeight: 'bold', marginBottom: 30, textAlign: 'center' },
  input: { borderWidth: 1, borderColor: '#ddd', padding: 15, borderRadius: 8, marginBottom: 15, fontSize: 16 },
  button: { backgroundColor: '#007AFF', padding: 15, borderRadius: 8, alignItems: 'center' },
  buttonText: { color: '#fff', fontSize: 16, fontWeight: 'bold' }
});

🎯 Tips & Best Practices

1. UX Optimization: Always set keyboardType="numeric" for phone and OTP inputs. This immediately opens the number pad, providing a frictionless experience for mobile users.
2. Security Considerations: For the sake of clarity, this tutorial makes API calls directly from the React Native app. For a production environment, you should build a simple proxy backend (using Express, Next.js, etc.) to store your API key securely out of reach from client-side users.

🚀 Conclusion

Adding SMS authentication to your React Native app doesn't have to be a multi-day ordeal.

EasyAuth(이지어스) is the perfect tool used in this example, tailored specifically for indie developers, side projects, and startup MVPs.

• Zero paperwork: No business registration required.
• Automatic sender ID: No need to go through caller ID pre-registration.
• Highly affordable: Only 15~25 KRW per message, which is nearly half the cost of traditional providers.
• 10 Free Credits: Get 10 test messages instantly upon sign-up to try out the code above!

Stop wasting days waiting for paperwork approvals. Sign up for EasyAuth and implement SMS OTP in your Expo app within 5 minutes today!