Closures

What is a closure?

A closure is created when an inner function remembers the variables from its outer environment, even after the outer function has finished executing.

In simple terms:

“A closure is a function that remembers where it was created.”

---

Concrete example: timer

A closure can be used to create a timer that keeps track of the number of seconds elapsed.

function startTimer() {
  let seconds = 0;

  return function() {
    seconds++;
    console.log("Elapsed time:", seconds, "s");
  };
}

const timer = startTimer();

timer(); // Elapsed time: 1 s
timer(); // Elapsed time: 2 s
timer(); // Elapsed time: 3 s

---

Practical uses of closures

• Preserve state across multiple calls (e.g. counter, timer).
• Create private variables not accessible from the outside.
• Avoid global variables → better encapsulation.
• Very useful in callbacks and functional programming.

---

Example: private variables

A closure can protect sensitive data (like a bank account balance) by making it accessible only through dedicated methods.

function createBankAccount() {
  let balance = 0; // private variable

  return {
    deposit(amount) {
      balance += amount;
      console.log("New balance:", balance);
    },
    withdraw(amount) {
      if (amount <= balance) {
        balance -= amount;
        console.log("Withdrawal successful, balance:", balance);
      } else {
        console.log("Insufficient funds");
      }
    }
  };
}

const account = createBankAccount();
account.deposit(100); // New balance: 100
account.withdraw(30); // Withdrawal successful, balance: 70
// console.log(account.balance); ❌ not directly accessible

---

Pitfalls to know

• Too many unnecessary closures can consume memory.
• They can create hidden references that prevent the garbage collector from freeing variables.
• Used wisely, they make code safer and better structured.

---

Summary

A closure = a function + its environment.
It’s one of the pillars of modern JavaScript (used in callbacks, arrow functions, frameworks like React, etc.).