Detect whether a web3 connection is available

web3.js

// Adapted from https://github.com/MetaMask/faq/blob/master/DEVELOPERS.md#partly_sunny-web3---ethereum-browser-environment-check
window.addEventListener('load', function() {

  // Checking if Web3 has been injected by the browser (Mist/MetaMask)
  if (typeof web3 !== 'undefined') {
    // Use Mist/MetaMask's provider
    window.web3 = new Web3(web3.currentProvider);
  } else {
    // fallback - use your fallback strategy (local node / hosted node + in-dapp id mgmt / fail)
    window.web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:8545"));
  }

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