gistfile1.js

  var account = (function(startingBalance) {

    var balance = startingBalance;

    function getBalance() { 
      return balance; 
    }

    function deposit(amount) {
      balance += amount;
    }

    function withdraw(amount) {
      balance -= amount;
    }

    return { 
      getBalance: getBalance,
      deposit: deposit,
      withdraw: withdraw
    };

  });

  var accountOne = account(0);
  var accountTwo = account(100);

  console.log(accountOne.getBalance()); // 0
  console.log(accountTwo.getBalance()); // 100

  accountOne.deposit(50);

  console.log(accountOne.getBalance()); // 50
  console.log(accountTwo.getBalance()); // 100