Summary - Closures

JavaScript course · Intermediate · Lesson 18/134

Build a bank account with private variables using closures and IIFE.

Summary - Closures

In this group you learned what a closure is, how to build private state with factory functions, how to create isolated scopes with IIFEs, how to design a public API with the module pattern, and how to avoid the loop closure pitfall.

Now we put it all together: a bank account that keeps a private balance and exposes only deposit, withdraw and getBalance methods.

Exercise Write a bank account

Define a function called createAccount(initial). Inside it keep a private balance with let balance = initial. The returned object should have three methods: deposit(amount) increments and returns the balance, withdraw(amount) decrements and returns the balance, getBalance() returns the balance. Then create an account with createAccount(100), and log deposit(50) and getBalance() calls.

Show example solution

function createAccount(initial) {
  let balance = initial;
  return {
    deposit: function(amount) { balance += amount; return balance; },
    withdraw: function(amount) { balance -= amount; return balance; },
    getBalance: function() { return balance; }
  };
}

const acc = createAccount(100);
console.log(acc.deposit(50));
console.log(acc.getBalance());

Exercise Add overdraft protection

Take the createAccount definition below and update the withdraw method: if the balance is insufficient, throw new Error("Insufficient balance"). Otherwise decrement and return the balance. The check should start with if (balance < amount).

Show example solution

function createAccount(initial) {
  let balance = initial;
  return {
    deposit: function(amount) { balance += amount; return balance; },
    withdraw: function(amount) {
      if (balance < amount) {
        throw new Error("Insufficient balance");
      }
      balance -= amount;
      return balance;
    },
    getBalance: function() { return balance; }
  };
}

const acc = createAccount(100);
console.log(acc.withdraw(50));

Test yourself

Match the concepts

closure

Remembers captured variables
IIFE

Runs immediately
module pattern

Guards private state
factory function

Builds functions
let in loop

Avoids the pitfall

Read the balance

function createAccount(initial) {
  let balance = initial;
  return {
    getBalance: function() { return ____; }
  };
}

Show answer

function createAccount(initial) {
  let balance = initial;
  return {
    getBalance: function() { return balance; }
  };
}

Summary - Closures