Closures

What is a Closure?

A closure is a function that "remembers" its lexical scope even when the function is executed outside that scope. In simpler terms: a closure gives you access to an outer function's scope from an inner function.

Basic Closure Example

function outer() {
  let message = "Hello from outer!";
  
  function inner() {
    console.log(message); // inner remembers 'message'
  }
  
  return inner;
}

let fn = outer();
fn(); // "Hello from outer!"

Practical Use Cases

• Data privacy — create private variables
• Function factories — create customized functions
• Event handlers — preserve state in callbacks

Private Variables with Closures

function createCounter() {
  let count = 0; // private variable
  
  return {
    increment: function() { count++; },
    decrement: function() { count--; },
    getCount: function() { return count; }
  };
}

let counter = createCounter();
counter.increment();
counter.increment();
console.log(counter.getCount()); // 2
// counter.count is not accessible

Common Mistakes

• Creating closures in loops without capturing values correctly (using let fixes this)
• Memory leaks from closures holding references to large objects
• Assuming closures copy values (they reference them)

Quick Quiz

Summary

• A closure is a function that remembers its lexical scope
• Every function in JavaScript creates a closure
• Closures enable data privacy and function factories
• Use closures to create private variables
• Be careful with closures in loops (use let)