Ethereum JSON-RPC Middleware for json-rpc-engine

import { JsonRpcEngine } from 'json-rpc-engine';
import { createFetchMiddleware, createBlockRefMiddleware, createChainIdMiddleware } from 'eth-json-rpc-middleware';

// Mock RPC service for createFetchMiddleware (required since v18.0.0)
const mockRpcService = {
  provider: {
    request: async ({ method, params }) => {
      console.log(`Mock RPC request: ${method} ${JSON.stringify(params)}`);
      // In a real application, this would forward to an actual RPC endpoint
      // using a provider from @metamask/network-controller or similar.
      if (method === 'eth_chainId') return '0x1'; // Mainnet
      if (method === 'eth_blockNumber') return '0x' + (123456789).toString(16);
      if (method === 'eth_sendRawTransaction') return '0xmockTxHash';
      return null; // Handle other methods as needed
    },
  },
  // Other properties like 'currentChainId' might be expected by some versions/integrations
};

async function setupAndUseRpcEngine() {
  const engine = new JsonRpcEngine();

  // Add middleware
  engine.add(createBlockRefMiddleware());
  engine.add(createChainIdMiddleware('0x1')); // Example: force chainId to Ethereum Mainnet
  engine.add(createFetchMiddleware({ rpcService: mockRpcService }));

  // Example request
  try {
    const response = await engine.handle({ id: 1, jsonrpc: '2.0', method: 'eth_blockNumber' });
    console.log('eth_blockNumber response:', response.result);

    const chainIdResponse = await engine.handle({ id: 2, jsonrpc: '2.0', method: 'eth_chainId' });
    console.log('eth_chainId response:', chainIdResponse.result);

    // Example of a request that might pass through the fetch middleware
    const sendTxResponse = await engine.handle({
      id: 3,
      jsonrpc: '2.0',
      method: 'eth_sendRawTransaction',
      params: ['0xmockrawtransactiondata'],
    });
    console.log('eth_sendRawTransaction response:', sendTxResponse.result);

  } catch (error) {
    console.error('RPC Error:', error);
  }
}

setupAndUseRpcEngine();