addnetwork.js

 var eth;
        var isTestnet = 'false';

        async function addNetwork(type) {

            if (type === 'web3') {
                if (typeof ethereum !== 'undefined') {
                    eth = new Web3Eth(ethereum);
                } else if (typeof web3 !== 'undefined') {
                    eth = new Web3Eth(web3.givenProvider);
                } else {
                    eth = new Web3Eth(ethereum);
                }
            }

            if (typeof eth !== 'undefined') {
                var network = 0;
                network = await eth.net.getId();
                netID = network.toString();
                var params;
                if (isTestnet == "false") {
                    if (netID == "137") {
                        alert("Polygon Network has already been added to Metamask.");
                        return;
                    } else {
                        params = [{
                            chainId: '0x89',
                            chainName: 'Matic Mainnet',
                            nativeCurrency: {
                                name: 'MATIC',
                                symbol: 'MATIC',
                                decimals: 18
                            },
                            rpcUrls: ['https://polygon-rpc.com/'],
                            blockExplorerUrls: ['https://polygonscan.com/']
                        }]
                    }
                } else {
                    if (netID == "80001") {
                        alert("Polygon Mumbai Network has already been added to Metamask.");
                        return;
                    } else {
                        params = [{
                            chainId: '0x13881',
                            chainName: 'Polygon Testnet',
                            nativeCurrency: {
                                name: 'MATIC',
                                symbol: 'MATIC',
                                decimals: 18
                            },
                            rpcUrls: ['https://rpc-mumbai.maticvigil.com/'],
                            blockExplorerUrls: ['https://mumbai.polygonscan.com/']
                        }]
                    }
                }

                window.ethereum.request({ method: 'wallet_addEthereumChain', params })
                    .then(() => console.log('Success'))
                    .catch((error) => console.log("Error", error.message));
            } else {
                alert('Unable to locate a compatible web3 browser!');
            }
        }