Fix: MetaMask SDK React Native Storage Crash in Next.js

The integration of the @metamask/sdk into a modern Next.js project is a common point of failure for Web3 architects. The primary issue is a “Dependency Leak”: the SDK, designed to be universal, includes hard references to @react-native-async-storage/async-storage. When Next.js’s Webpack or Turbo bundler attempts to resolve these native modules for a web-based environment, it triggers a fatal Module not found error, halting both development and production builds.

Diagnostic Error Trace

- error ./node_modules/@metamask/sdk/dist/browser/index.js
Module not found: Can't resolve '@react-native-async-storage/async-storage'

Possible causes:
1. The SDK is trying to initialize mobile storage in a browser environment.
2. Next.js Webpack is traversing the 'native' export path of a multi-platform package.

Immediate Fix: You must alias the React Native storage dependency to a web-native equivalent (like local-storage or a simple null object) and force Next.js to transpile the MetaMask SDK.

// next.config.mjs
/** @type {import('next').NextConfig} */
const nextConfig = {
  transpilePackages: ['@metamask/sdk'],
  webpack: (config) => {
    config.resolve.alias = {
      ...config.resolve.alias,
      '@react-native-async-storage/async-storage': false,
    };
    return config;
  },
};
export default nextConfig;

Architectural Breakdown: Cross-Platform Dependency Pollution

To understand why this happens, we must look at how modern “Universal” SDKs are built. MetaMask’s SDK uses a single codebase to support React, Vue, Angular, and React Native.

The “All-at-Once” Bundling Strategy

Many SDKs use a single entry point that dynamically selects the platform at runtime. However, static analyzers (like Webpack) see all import and require statements during the build phase.

• The Conflict: Even if the SDK has logic like if (isMobile) { require('react-native-storage') }, Webpack still tries to find that file in your node_modules. Since you are building a web app, you haven’t installed @react-native-async-storage/async-storage, leading to the resolution error.

SSR Module Isolation

Next.js compounds this problem. During Server-Side Rendering (SSR), the code runs in a Node.js environment. The MetaMask SDK might detect this as “not a browser” and try to fall back to a non-existent storage mechanism, causing “Window is not defined” or “Storage is not defined” errors during the initial page load.

Deep-Dive Analysis: The SDK Initialization Lifecycle

The MetaMask SDK initialization process involves setting up a CommunicationLayer and a StorageLayer.

1. The Storage Layer Conflict

The SDK attempts to persist the session (the pairing data) so users don’t have to scan the QR code every time they refresh the page. On the web, it should use localStorage. In React Native, it uses AsyncStorage. The bug occurs because the “Auto-Detection” logic in the SDK is often too aggressive, causing the bundler to get confused about which storage provider to package.

2. The Transpilation Gap

Because @metamask/sdk is distributed as high-level ESM (ES Modules), older bundlers or strict Next.js configurations might fail to parse the optional chaining or nullish coalescing operators inside the SDK’s distribution folder. This is why adding it to transpilePackages is a non-negotiable step.

3. IFrame and Popup Contexts

MetaMask SDK often opens an iframe or a popup to handle the “MMSDK” (MetaMask Mobile SDK) connection. These sub-windows have their own execution contexts. If your next.config.js isn’t correctly aliasing the storage, these sub-windows might crash silently, leading to a “Connection Timeout” in the main UI.

Technical Implementation: Implementing the Web-Only Storage Shim

We will create a robust configuration that satisfies both the Next.js compiler and the MetaMask SDK’s runtime requirements.

Step 1: Create a Mock Storage (The Shim)

Create a file at src/lib/metamask-storage-shim.js. This file mimics the API of AsyncStorage but uses the browser’s localStorage.

// src/lib/metamask-storage-shim.js
const storageMock = {
  getItem: async (key) => typeof window !== 'undefined' ? localStorage.getItem(key) : null,
  setItem: async (key, value) => typeof window !== 'undefined' ? localStorage.setItem(key, value) : null,
  removeItem: async (key) => typeof window !== 'undefined' ? localStorage.removeItem(key) : null,
};
export default storageMock;

Step 2: Configure next.config.mjs for Alias Injection

Direct Webpack to use our shim whenever it sees the React Native storage package.

// next.config.mjs
import path from 'path';
import { fileURLToPath } from 'url';

const __dirname = path.dirname(fileURLToPath(import.meta.url));

export default {
  transpilePackages: ['@metamask/sdk'],
  webpack: (config, { isServer }) => {
    if (!isServer) {
      config.resolve.alias['@react-native-async-storage/async-storage'] = path.resolve(
        __dirname,
        'src/lib/metamask-storage-shim.js'
      );
    } else {
      config.resolve.alias['@react-native-async-storage/async-storage'] = false;
    }
    return config;
  },
};

Step 3: Lazy-Load the SDK

Use dynamic imports to ensure the SDK is only loaded in the browser. This is the “Gold Standard” for Web3 Next.js development.

'use client';
import { useEffect, useState } from 'react';

export default function MetaMaskProvider({ children }) {
  const [sdk, setSdk] = useState(null);

  useEffect(() => {
    const initSDK = async () => {
      const { MetaMaskSDK } = await import('@metamask/sdk');
      const mmsdk = new MetaMaskSDK({
        dappMetadata: { name: "My Dapp", url: window.location.href },
      });
      setSdk(mmsdk);
    };
    initSDK();
  }, []);

  return <>{children}</>;
}

Production Prevention: Managing Universal SDKs in Web Frameworks

To prevent similar “Dependency Leaks” from other libraries (like WalletConnect or Firebase), follow these architectural principles.

1. Explicit Bundle Auditing

Use webpack-bundle-analyzer periodically. If you see @react-native or @expo packages in your web bundle, you have a dependency leak. This not only causes build errors but also bloats your bundle size with code that can never run.

2. The “Adapter” Pattern

Instead of calling third-party SDKs directly in your components, wrap them in an Adapter. This allows you to “Hide” the problematic dependencies behind a clean interface that you control. If the SDK updates and breaks the storage logic, you only have to fix it in the Adapter, not across 20 components.

3. CI/CD “No-Native” Linting

Add a simple bash script to your CI pipeline that greps your node_modules for “react-native” and checks if those modules are being imported in your dist folder. This acts as an early-warning system for dependency pollution.

Forensic Analysis: The Paradox of “Write Once, Run Anywhere”

The MetaMask SDK is a victim of its own ambition. By trying to support every platform with a single package, it creates a “Dependency Hell” for developers using specialized frameworks like Next.js.

The Problem of Transitive Dependencies

The most frustrating part of this bug is that you didn’t even ask for React Native. It was pulled in as a “Transitive Dependency.” In the world of Web3, where security and performance are critical, these hidden dependencies are more than just a nuisance—they are a liability.

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FAQ: MetaMask SDK and Next.js Recovery

1. Does this happen with Wagmi’s MetaMask connector?

No. Wagmi’s standard injected connector uses the browser-native window.ethereum and does not rely on the @metamask/sdk. This error is specific to developers using the SDK to enable “MetaMask Mobile” deep-linking or QR-code support on the web.

2. Why does transpilePackages fix the ‘optional chaining’ error?

Next.js’s default compiler (SWC) doesn’t always process files inside node_modules to save time. By adding a package to transpilePackages, you are telling Next.js “Treat this folder as if it were my own source code,” which ensures all modern JS syntax is transpiled to a version the browser (and the server) can understand.

3. Can I use the ‘browser’ field in package.json to fix this?

In theory, yes. If the library author correctly configured the browser field to point to a different file, Webpack would use it. Unfortunately, many Web3 libraries have incomplete browser mappings, necessitating the manual alias fix in next.config.js.

4. How do I handle ‘process is not defined’ in the MetaMask SDK?

This is another common Next.js error. Use the DefinePlugin in Webpack or a simple global.process = { env: {} } shim in your client-side entry point to satisfy the SDK’s internal environment checks.