eth_chainId | Polygon

Polygon API method that returns the current chain ID. Chain ID is used to sign replay-protected transactions and generally verify if a network is the desired one. It was introduced in EIP-155.

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Parameters

• none

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Response

• quantity — EIP-155 Chain ID

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eth_chainId code examples

const Web3 = require("web3");

const NODE_URL = "CHAINSTACK_NODE_URL";

const web3 = new Web3(NODE_URL);



async function getChainId() {

  const chain = await web3.eth.getChainId();

  console.log(chain);

}



getChainId();

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Use case

One possible use case for the eth_chainId method in Polygon is to determine which chain a user is connected to when using a DApp with the MetaMask browser extension. This can be useful to provide a customized user experience based on the specific chain that the user is interacting with.

Here is an example of how the eth_chainId method might be used in this context with MetaMask. Keep in mind that the Chain ID is returned as a hexadecimal value:

// Check which network is selected on MetaMask

async function checkChain() {

  

    // Declare the desired chain ID to match

    const desiredChainId = '0x89'; // Polygon mainnet chain ID

  

    // Retrieve the current Chain ID selected by the user 

    ethereum.request({

        method: 'eth_chainId'

    }).then(chainId => {

      

        // compare the actual chain ID it to the declared chain ID 

        if (chainId !== desiredChainId) {

          

            console.log(`You are currently on the wrong network. Please switch to the mainnet.`);

            // Call the promtSwitch function

            promtSwitch()

          

        } else {

            console.log("This is the correct network")

        }

    })

}

// Prompt user to switch to a determined network based on the chain ID

async function promtSwitch() {

    await window.ethereum.request({

        method: 'wallet_switchEthereumChain',

        params: [{

            chainId: '0x89' // chainId must be in HEX with 0x in front

        }],

    });

}

In this case, the checkChain function is called to get the current chain ID using the Ethereum object provided by MetaMask. The chain ID is then checked to determine if the user is connected to the Polygon mainnet and calls the promtSwitch function to prompt the user to switch chain in case the chain ID returned does not match the desiredChainId constant.