eth_chainId - Polygon

Example code for the eth_chainId json-rpc method. Сomplete guide on how to use eth_chainId json-rpc in GetBlock.io Web3 documentation.

The eth_chainId method returns the current chain ID used for signing replay-protected transactions (EIP-155). This method is essential for applications that need to verify they are connected to the correct network and for properly signing transactions.

Parameters

• None

Request

cURL

cURL

curl --location --request POST 'https://go.getblock.io/<ACCESS-TOKEN>/' \
--header 'Content-Type: application/json' \
--data-raw '{
    "jsonrpc": "2.0",
    "method": "eth_chainId",
    "params": [],
    "id": "getblock.io"
}'

Axios

Request

Rust

Response

{
    "jsonrpc": "2.0",
    "id": "getblock.io",
    "result": "0x89"
}

Response Parameters

Field	Type	Description
jsonrpc	string	JSON-RPC version (2.0)
id	string	Request identifier
result	string	Chain ID in hexadecimal format (0x89 = 137 for Polygon Mainnet)

Use Case

The eth_chainId method is essential for:

• Network verification: Confirm connection to the correct network before operations

• Transaction signing: Include correct chain ID for EIP-155 replay protection

• Multi-chain applications: Dynamically detect and switch between networks

• Wallet integration: Verify network configuration matches user expectations

• Security: Prevent cross-chain replay attacks

Example: Network Verification

JavaScript (example)

const POLYGON_MAINNET_CHAIN_ID = 137;
const POLYGON_AMOY_CHAIN_ID = 80002;

async function verifyNetwork(provider) {
    const chainId = await provider.send('eth_chainId', []);
    const numericChainId = parseInt(chainId, 16);
    
    if (numericChainId === POLYGON_MAINNET_CHAIN_ID) {
        console.log('Connected to Polygon Mainnet');
        return 'mainnet';
    } else if (numericChainId === POLYGON_AMOY_CHAIN_ID) {
        console.log('Connected to Polygon Amoy Testnet');
        return 'testnet';
    } else {
        throw new Error(`Unexpected chain ID: ${numericChainId}`);
    }
}

Error Handling

Status Code	Error Message	Cause
403	Forbidden	Missing or invalid ACCESS-TOKEN
-32600	Invalid Request	Malformed request body

Web3 Integration

Ethers.js

Ethers.js

import { ethers } from 'ethers';

const provider = new ethers.JsonRpcProvider('https://go.getblock.io/<ACCESS-TOKEN>/');

// Using network property
const network = await provider.getNetwork();
console.log('Chain ID:', network.chainId);

// Using raw RPC call
const chainIdHex = await provider.send('eth_chainId', []);
console.log('Chain ID (hex):', chainIdHex);

Viem

Web3.js