How to verify contract on Etherscan, PolygonScan, Optimism Etherscan, Arbiscan, or Snowtrace programmatically with Node.js. This is mostly because the API documentation is poorly done and does not showcase a way to actually do this. I figured it out so i figure others would like to know if using the API

index.js

const EXPLORER = {
  ethereum: {
    api: 'https://api.etherscan.io/api',
    key: '<API_KEY_HERE>', // Etherescan API key works for ethereum and rinkeby, will need to do this for each API
  },
  rinkeby: {
    api: 'https://api-rinkeby.etherscan.io/api',
    key: '',
  },
 

  polygon: {
    api: 'https://api.polygonscan.com/api',
    key: '',
  },
  mumbai: {
    api: 'https://api-testnet.polygonscan.com/api',
    key: '',
  },
  
  // Optimism Does not support contract validation from API
  optimism: {
    api: null,
    key: '',
  },
  kovanOptimism: {
    api: null,
    key: '',
  },

  arbitrum: {
    api: 'https://api.arbiscan.io/api',
    key: '',
  },
  rinkebyArbitrum: {
    api: 'https://api-testnet.arbiscan.io/api',
    key: '',
  },

  avalanche: {
    api: 'https://api.snowtrace.io/api',
    key: '',
  },
  testnetAvalanche: {
    api: 'https://api-testnet.snowtrace.io/api',
    key: '',
  },
};

async function verifyContract(network) {
  const explorer = EXPLORER[network];
  const contractSourceCode = 'pragma .......'
  const contractAddress = '0xAnsfb212n12...'

  if (!explorer.api) {
    return {
      status: false,
      msg: `Contract validation not available for ${network}`,
    };
  }

  var formBody = [];
  
  // In order for this to work EVERY key asked for in the API must be included - even if using default.
  const data = {
    apikey: explorer.key,
    module: 'contract',
    action: 'verifysourcecode',
    contractaddress: contractAddress,
    sourceCode: contractSourceCode,
    codeformat: 'solidity-single-file',
    contractname: `MyContract`, // should match the `contract` name in source code
    optimizationUsed: 1,
    compilerversion: 'v0.8.9+commit.e5eed63a', // pick yours from the list of available compilers found in docs for API
    constructorArguements: '',
    runs: 200,
    evmversion: 'default',
    licenseType: 1, // Choose whichever option you want - refer to docs for API
  };
  
  for (var property in data) {
    var encodedKey = encodeURIComponent(property);
    var encodedValue = encodeURIComponent(data[property]);
    formBody.push(encodedKey + '=' + encodedValue);
  }
  formBody = formBody.join('&');

  const response = await fetch(explorer.api, {
    method: 'POST',
    headers: {
      'Content-Type': 'application/x-www-form-urlencoded',
    },
    body: formBody,
  })
    .then((res) => res.json())
    .then((res) => {
      if (+res.status === 1) {
        return {
          status: true,
          msg: `Contract verified!`,
          more: res,
        };
      }

      return {
        status: false,
        msg: `Contract failed to verify!`,
        more: res,
      };
    })
    .catch((e) => {
      console.error(e);
      return {
        status: false,
        msg: `Contract failed to verify!`,
        more: e,
      };
    });

  return response;
}