Closures - Exercise

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Closures - Exercise


Duration: 15 to 25 minutes.

In the simple bank account application you will write in this exercise, you will use closures as a way to maintain state (to keep track of the account balance), to offer external code a way to get and set information (make a deposit or get the current balance), and to protect data (like the balance) from inappropriate tampering.

  1. Open AdvancedFunctions/Exercises/closures.html for editing.
  2. Write the body of function bankAccount as indicated in the comments:
    • Add local variables balance and ownerName
    • Return an object with fields for withdrawal(withdrawalAmount), deposit(depositAmount), getBalance(), and getOwnerName()
    • Add validation to ensure only appropriate withdrawals and deposits are allowed
  3. Test your solution in a browser.


<!DOCTYPE html>
	<meta charset="utf-8">
		function bankAccount(ownerName) {
			var balance = 0;
			var ownerName = ownerName;
			return {
				withdrawal: function(withdrawalAmount) {
					if (withdrawalAmount > 0 && withdrawalAmount <= balance) {
						balance -= withdrawalAmount;
				deposit: function(depositAmount) {
					if (depositAmount > 0) {
						balance += depositAmount;
				getBalance: function() {
					return balance;
				getOwnerName: function() {
					return ownerName;
		var myAccount = bankAccount('Jane Doe');
		document.write('Current balance: $' + myAccount.getBalance());
		document.write('<br>Current balance: $' + myAccount.getBalance());
		myAccount.withdrawal(2000); // this shouldn't work, since 2000 > current balance
		document.write('<br>Current balance: $' + myAccount.getBalance());
		myAccount.deposit(-2); // this shouldn't work, since -2 <= 0
		document.write('<br>Current balance: $' + myAccount.getBalance());
		document.write('<br>Account owner: ' + myAccount.getOwnerName());

Code Explanation

In function bankAccount we set local variables balance to 0 and ownerName to the string argument supplied when the function is first called.

Function bankAccount returns an object where each field is a function and thus a closure: this ensures that external code can get (myAccount.getBalance(), myAccount.getOwnerName()) and set (myAccount.deposit(100), myAccount.withdrawal(75)) properties, but external code cannot directly access nor modify properties.