How to handle chain changes in my dApp with MetaMask?

Understanding Chain Changes in dApps

When developing or interacting with decentralized applications (dApp), one of the most common challenges users and developers face is handling chain changes. MetaMask, being one of the most widely used cryptocurrency wallets, allows users to switch between different blockchain networks. However, this can cause disruptions if a dApp is not configured properly to detect and respond to such chain changes. This includes scenarios like switching from Ethereum Mainnet to Binance Smart Chain or any testnet environment.

The core issue revolves around ensuring that your dApp reacts correctly when the network selected in MetaMask changes. This is critical for maintaining user experience and preventing transaction errors due to incorrect chain IDs or RPC endpoints.

Detecting Network Changes in MetaMask

MetaMask provides an event listener called chainChanged, which is triggered whenever the user switches to a different network. To handle this, you need to implement a chainChanged event listener within your dApp's JavaScript code.

Here’s how you can set it up:

• Import Web3 or use the injected provider from window.ethereum.
• Add an event listener for chainChanged:

  window.ethereum.on('chainChanged', handleChainChanged);

• Define the handleChainChanged function to reload or reconfigure the dApp based on the new chain ID.

This ensures that your dApp always knows what network the user is currently connected to and can adapt accordingly.

Handling Chain IDs and Network Identifiers

Each blockchain network has a unique identifier known as the chain ID. For example, Ethereum Mainnet uses chain ID 1, while Rinkeby uses 4. When a user switches chains, the chainChanged event returns the new chain ID in hexadecimal format.

To ensure compatibility, your dApp should:

• Convert the returned chain ID from hexadecimal to decimal using parseInt(chainId, 16).
• Compare the resulting value against a list of supported chain IDs.
• Display appropriate UI feedback or redirect the user if the selected chain is unsupported.

If the chain is not supported, your dApp should prompt the user to switch back or provide instructions on how to add custom networks through MetaMask.

Updating Provider and Reconnecting Contracts

After detecting a chain change, it's essential to update your provider and reconnect any smart contract instances. Failure to do so may result in incorrect data retrieval or failed transactions.

Steps to follow:

• Create a new instance of Web3 using window.ethereum after the chain change.
• Reconnect all contract instances using the updated provider.
• Re-fetch account balances and contract states to reflect the current network.

This process ensures that all interactions occur on the correct chain and that your dApp remains responsive and accurate regardless of network switches.

Prompting Users to Add Custom Chains

Sometimes, your dApp might require users to connect to a network that isn't available by default in MetaMask. In such cases, you can prompt them to add a custom network using the wallet_addEthereumChain method.

Here’s how you can trigger this action:

• Prepare the RPC URL, chain ID, currency symbol, and block explorer URL for the target network.
• Call the following method:

  await window.ethereum.request({  method: 'wallet_addEthereumChain',  params: [{
  
  
  
  
  
  chainId: '0x...',
  rpcUrls: ['https://...'],
  chainName: 'Custom Network',
  nativeCurrency: {
    name: 'Token',
    symbol: 'TKN',
    decimals: 18
  },
  blockExplorerUrls: ['https://...']
  }]});

This will open a MetaMask confirmation dialog allowing the user to add the network securely.

Frequently Asked Questions

How often does the chainChanged event fire?

The chainChanged event fires every time the user manually switches networks in MetaMask or when another dApp triggers a network change. It does not fire during initial page load but only when the network changes post-initialization.

Can I programmatically switch chains in MetaMask?

Yes, you can request a chain switch using the wallet_switchEthereumChain method. You must pass the desired chain ID in hexadecimal format. If the chain is not already added, MetaMask will prompt the user to add it first.

Why doesn’t my dApp detect chain changes immediately?

Your dApp must actively listen for the chainChanged event using window.ethereum.on('chainChanged', callback). If this listener is not registered at runtime or gets removed during execution, the dApp won’t detect chain changes.

What happens if a user selects an unsupported chain?

If your dApp doesn’t support the selected chain, it should display a warning message indicating that the current network is not supported. You can also provide a button to guide the user through adding the correct network or switching back to a supported one.