function sample(uint i, string memory s, Zombie storage _zombie)
    external view onlyOwner anotherModifier {
  /* ... */
  Zombie storage myZombie = xx;
}

contract OnlineStore {
  function buySomething() external payable {
    // Check to make sure 0.001 ether was sent to the function call:
    require(msg.value == 0.001 ether);
    // If so, some logic to transfer the digital item to the caller of the function:
    transferThing(msg.sender);
  }
}

using SafeMath for uint256;
// instead of x++ to prevent overflow;
x = x.add(1);

var myContract = new web3js.eth.Contract(myABI, myContractAddress);

// runs on the local node for view and pure functions
// don't cost any gas (read-only)
// user won't be prompted to sign a transaction with MetaMask
myContract.methods.myMethod(123).call();

// create a transaction and change data on the blockchain
// user need to pay gas
// Metamask prompt user to sign a transaction
myContract.methods.myMethod(123).send()

// helper function example
function myMethod(param) {
  return myContract.methods.myMethod(param).call();
}

// find web3 object at startup
window.addEventListener('load', function() {

  // 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 {
    // Handle the case where the user doesn't have Metamask installed
    // Probably show them a message prompting them to install Metamask
  }

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

})

// keep checking account and update if it is switched
var accountInterval = setInterval(function() {
  // Check if account has changed
  if (web3.eth.accounts[0] !== userAccount) {
    userAccount = web3.eth.accounts[0];
    // Call a function to update the UI with the new account
    getZombiesByOwner(userAccount)
    .then(displayZombies);
  }
}, 100);