eth_chainId | Polygon

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The eth_chainId method is a fundamental Ethereum-compatible JSON-RPC API call used across Polygon networks to retrieve the current chain’s unique identifier. This identifier, defined under EIP-155, plays a critical role in transaction signing, network validation, and ensuring replay protection across blockchain networks. For developers building decentralized applications (DApps) on Polygon, accurately detecting the user's active network using eth_chainId ensures security, compatibility, and a seamless user experience.

Whether you're deploying smart contracts, managing wallet integrations, or validating network conditions, understanding how to implement eth_chainId effectively is essential. This guide explores its functionality, use cases, response format, and practical code implementation—especially within popular tools like MetaMask.

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What Is eth_chainId?

The eth_chainId method returns the current chain ID of the Ethereum Virtual Machine (EVM)-compatible network that a node or client is connected to. In the context of Polygon, this allows developers to confirm whether their application is interacting with the Polygon Mainnet, Mumbai Testnet, or another EVM-equivalent sidechain.

Chain IDs were introduced through EIP-155 to prevent transaction replay attacks between different chains. For example, without chain isolation, a transaction valid on Ethereum could be maliciously rebroadcast on another network like Polygon. The inclusion of the chain ID in transaction signatures eliminates this risk.

This method requires no input parameters and returns a hexadecimal value representing the chain ID.

Parameters

None

The eth_chainId method does not accept any arguments. It simply queries the connected node for the current network’s chain ID.

Response Format

This response is part of the standard Ethereum JSON-RPC specification and is returned as a JSON object. Here's an example:

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": "0x89"
}

In this case, 0x89 translates to 137 in decimal—the official chain ID for Polygon Mainnet.

Other common Polygon-related chain IDs include:

Accurate detection of these values helps prevent users from interacting with DApps on unsupported or incorrect networks.

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Practical Use Case: Network Validation in DApps

One of the most common use cases for eth_chainId is verifying which blockchain network a user is currently connected to—especially when using browser wallets like MetaMask.

Imagine a user visiting your DApp built on Polygon but accidentally remains on the Ethereum network. Without proper checks, transactions may fail or interact with incorrect contract addresses. By calling eth_chainId, your app can detect mismatches and prompt the user to switch networks automatically.

Example: Detecting and Switching Networks with MetaMask

Below is a real-world JavaScript implementation that uses eth_chainId to check the user's current network and prompt them to switch if necessary:

// Check which network is selected in MetaMask
async function checkChain() {
  const desiredChainId = '0x89'; // Polygon Mainnet

  try {
    // Request current chain ID from MetaMask
    const chainId = await window.ethereum.request({ method: 'eth_chainId' });

    if (chainId !== desiredChainId) {
      console.log('You are on the wrong network. Switching to Polygon Mainnet...');
      await promptSwitch();
    } else {
      console.log('Connected to the correct network: Polygon Mainnet');
    }
  } catch (error) {
    console.error('Error checking network:', error);
  }
}

// Prompt user to switch to Polygon Mainnet
async function promptSwitch() {
  try {
    await window.ethereum.request({
      method: 'wallet_switchEthereumChain',
      params: [{ chainId: '0x89' }],
    });
  } catch (error) {
    // If the network isn't added, add it manually
    if (error.code === 4902) {
      try {
        await window.ethereum.request({
          method: 'wallet_addEthereumChain',
          params: [
            {
              chainId: '0x89',
              chainName: 'Polygon Mainnet',
              nativeCurrency: {
                name: 'MATIC',
                symbol: 'MATIC',
                decimals: 18,
              },
              rpcUrls: ['https://polygon-rpc.com/'],
              blockExplorerUrls: ['https://polygonscan.com/'],
            },
          ],
        });
      } catch (addError) {
        console.error('Failed to add Polygon network:', addError);
      }
    } else {
      console.error('Failed to switch networks:', error);
    }
  }
}

// Call function on page load
checkChain();

This script first retrieves the current chain ID and compares it against the expected value (0x89). If there's a mismatch, it attempts to switch the user to Polygon Mainnet using wallet_switchEthereumChain. If the network isn’t already configured in MetaMask, it adds it via wallet_addEthereumChain.

This kind of proactive validation improves UX and reduces errors caused by misconfigured wallets.

Core Keywords

To ensure optimal search visibility and alignment with user intent, here are the core SEO keywords naturally integrated throughout this article:

These terms reflect high-intent searches from developers working on blockchain integration, wallet connectivity, and cross-network compatibility.

Frequently Asked Questions (FAQ)

What does eth_chainId return?

The eth_chainId method returns the current blockchain’s unique identifier as a hexadecimal string. For example, Polygon Mainnet returns "0x89", while Ethereum Mainnet returns "0x1".

How is chain ID different from network ID?

While both identify networks, the chain ID is used specifically for transaction signature replay protection (per EIP-155), whereas network ID is used internally by nodes for peer-to-peer networking purposes. They often have the same value but serve different functions.

Can eth_chainId be used on testnets?

Yes. You can use eth_chainId on any EVM-compatible testnet, including Polygon Mumbai (0x13881) and Amoy (0x13882). Always verify the expected chain ID for your target test environment.

Why is checking chain ID important for DApps?

Checking the chain ID prevents users from performing actions on unintended networks, reducing errors, failed transactions, and potential loss of funds due to misdirected interactions.

Does MetaMask support eth_chainId?

Yes, MetaMask fully supports eth_chainId through its Ethereum provider (window.ethereum). It's one of the primary methods used by DApps to detect a user's active network.

What happens if the wrong chain ID is used?

Using an incorrect chain ID during transaction signing can result in invalid transactions or replay attacks. Always ensure your application validates the correct chain before proceeding with sensitive operations.

With growing adoption of multi-chain architectures, tools like eth_chainId become indispensable for robust DApp development. Whether you're verifying user connections or enabling automatic network switching, this simple yet powerful method enhances both security and usability.

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