getWeb3.js

import Web3 from 'web3'
let getWeb3 = () => {
  return new Promise(function (resolve, reject) {
    // Wait for loading completion to avoid race conditions with web3 injection timing.
    window.addEventListener('load', function () {
      var results
      var web3 = window.web3

      // Checking if Web3 has been injected by the browser (Mist/MetaMask)
      if (typeof web3 !== 'undefined') {
        // Use Mist/MetaMask's provider.
        web3 = new window.Web3(web3.currentProvider)
        web3.version.getNetwork((err, netId) => {
          let netIdName, trustApiName, explorerUrl;
          console.log('netId', netId);
          switch (netId) {
            case "1":
              netIdName = 'Foundation'
              trustApiName = 'api'
              explorerUrl = 'https://etherscan.io'
              console.log('This is Foundation', netId)
              break;
            case "3":
              netIdName = 'Ropsten'
              trustApiName = 'ropsten'
              explorerUrl = 'https://ropsten.etherscan.io'
              console.log('This is Ropsten', netId)
              break;
            case "4":
              netIdName = 'Rinkeby'
              trustApiName = 'rinkeby'
              explorerUrl = 'https://rinkeby.etherscan.io'
              console.log('This is Rinkeby', netId)
              break;
            case "42":
              netIdName = 'Kovan'
              trustApiName = 'kovan'
              explorerUrl = 'https://kovan.etherscan.io'
              console.log('This is Kovan', netId)
              break;
            case "99":
              netIdName = 'POA Core'
              trustApiName = 'poa'
              explorerUrl = 'https://poaexplorer.com'
              console.log('This is Core', netId)
              break;
            case "77":
              netIdName = 'POA Sokol'
              trustApiName = 'https://trust-sokol.herokuapp.com'
              explorerUrl = 'https://sokol.poaexplorer.com'
              console.log('This is Sokol', netId)
              break;
            default:
              netIdName = 'Unknown'
              console.log('This is an unknown network.', netId)
          }
          document.title = `${netIdName} - MultiSender dApp`
          var defaultAccount = web3.eth.defaultAccount || null;
          if(defaultAccount === null){
            reject({message: 'Please unlock your metamask and refresh the page'})
          }
          results = {
            web3Instance: web3,
            netIdName,
            netId,
            injectedWeb3: true,
            defaultAccount,
            trustApiName,
            explorerUrl
          }
          resolve(results)
        })

        console.log('Injected web3 detected.');

      } else {
        // Fallback to localhost if no web3 injection.
        const errorMsg = `Metamask is not installed. Please go to
        https://metamask.io and return to this page after you installed it`
        reject({message: errorMsg})
        console.log('No web3 instance injected, using Local web3.');
        console.error('Metamask not found'); 
      }
    })
  })
}

export default getWeb3