NextAuth.js Error: "Unexpected token '<' "<!DOCTYPE ..." is not valid JSON" - A Guide to Troubleshooting
The Problem:
You're trying to use NextAuth.js in your Next.js application, but you encounter an error that reads "[next-auth][error][CLIENT_FETCH_ERROR] Unexpected token '<' "<!DOCTYPE ..." is not valid JSON". This error means that NextAuth.js is unable to parse the response it receives from your authentication provider. This is usually due to a mismatch between the expected JSON format and the actual HTML content returned.
Understanding the Scenario:
This error typically arises when a third-party authentication provider, such as Google or GitHub, returns an HTML response instead of the expected JSON data. This can happen if the provider is facing temporary issues, or if there's a misconfiguration in the way your application interacts with the provider.
Let's Break Down the Code:
The error message points to a specific problem: the server is not receiving a valid JSON response. Let's take a look at a typical example:
// pages/api/auth/[...nextauth].js
import NextAuth from 'next-auth';
import GoogleProvider from 'next-auth/providers/google';
export default NextAuth({
providers: [
GoogleProvider({
clientId: process.env.GOOGLE_CLIENT_ID,
clientSecret: process.env.GOOGLE_CLIENT_SECRET,
}),
],
});
In this code, NextAuth.js is configured to use the Google provider for authentication. If the Google server returns HTML instead of JSON, it'll trigger the error.
Troubleshooting Steps:
- Verify Provider Configuration: Double-check your authentication provider settings. Ensure that the
clientIdandclientSecretare correctly configured. - Check Provider Documentation: Consult the documentation of your authentication provider for any known issues or specific requirements.
- Network Inspection: Use your browser's developer tools to inspect the network requests and responses. Check if the response from your authentication provider is indeed HTML instead of JSON.
- Check Provider Status: Check if the authentication provider is experiencing any known outages or service disruptions. You can consult their status pages or community forums for updates.
- Handle Errors Gracefully: Implement error handling within your NextAuth.js configuration to catch any unexpected responses and provide appropriate feedback to the user. You can do this using the
callbacksoption:
export default NextAuth({
// ... other configuration
callbacks: {
jwt: async ({ token, user, account, profile, isNewUser }) => {
if (user) {
// ...
} else {
// Handle error gracefully
console.error('Authentication failed:', user, account, profile);
// ...
}
return token;
},
},
});
Additional Tips:
- Use the Correct Provider: Ensure you are using the correct provider for your authentication needs. For example, if you want to use Google OAuth, make sure you are using the
GoogleProviderfromnext-auth/providers/google. - Clear Cache: Clear your browser cache and cookies to ensure you are not dealing with stale data.
- Restart Server: If you're using a development server, try restarting it to see if that resolves the issue.
Conclusion:
This "Unexpected token '<'" error is a common issue in NextAuth.js that can often be resolved by double-checking your authentication provider configuration and understanding the nature of the response you're receiving. By implementing these troubleshooting steps and carefully inspecting the network requests, you can identify and resolve the problem, restoring your NextAuth.js integration and getting your application back on track.
Further Resources:
- NextAuth.js Documentation: Comprehensive documentation for NextAuth.js.
- Google OAuth Documentation: Detailed information about Google OAuth integration.
- GitHub OAuth Documentation: Documentation for GitHub OAuth authentication.
