web3 init.js

// When the page loads, we create a web3 instance and set a provider. We then set up the app
window.addEventListener("load", function() {
  // Is there an injected web3 instance?
  if (typeof web3 !== "undefined") {
    console.warn("Using web3 detected from external source like Metamask")
    // If there is a web3 instance(in Mist/Metamask), then we use its provider to create our web3object
    window.web3 = new Web3(web3.currentProvider)
  } else {
    console.warn("No web3 detected. Falling back to http://localhost:9545. You should remove this fallback when you deploy live, as it's inherently insecure. Consider switching to Metamask for development. More info here: http://truffleframework.com/tutorials/truffle-and-metamask")
    // fallback - use your fallback strategy (local node / hosted node + in-dapp id mgmt / fail)
    window.web3 = new Web3(new Web3.providers.HttpProvider("http://localhost:9545"))
  }
  // initializing the App
  window.App.start()
})