check-metamask.js

window.addEventListener('load', () => {
  // Checking if Web3 has been injected by the browser (Mist/MetaMask)
  if (typeof web3 !== 'undefined') {
    // Use Mist/MetaMask's provider
    web3js = new Web3(web3.currentProvider);
  } else {
    console.log('No web3? You should consider trying MetaMask!');
    // fallback - use your fallback strategy (local node / hosted node + in-dapp id mgmt / fail)
    web3js = new Web3(new Web3.providers.HttpProvider('http://localhost:8545'));
  }

  // Now you can start your app & access web3 freely:
  startApp();
});