check-network.js

if (web3js) {
  switch (web3js.version.network) {
    case '1':
      console.log('This is mainnet');
      break;
    case '2':
      console.log('This is the deprecated Morden test network.');
      break;
    case '3':
      console.log('This is the ropsten test network.');
      break;
    case '4':
      console.log('This is the Rinkeby test network.');
      break;
    case '42':
      console.log('This is the Kovan test network.');
      break;
    default:
      console.log('This is an unknown network.');
  }

  const desiredNetwork = 1;
  if (web3js.version.network !== desiredNetwork) {
    // ask user to switch to desired network
    console.log('Please switch to main network.');
  }
}