JavaScript Closures

What is a Closure?

A closure is a function that retains access to variables from its outer scope, even after the outer function has finished executing.

Basic Example

closure.js

function createCounter() {
  let count = 0;

  return {
    increment() {
      count++;
      return count;
    },
    getCount() {
      return count;
    }
  };
}

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2
console.log(counter.getCount()); // 2

Why Closures Matter

1. Data privacy - Create private variables
2. Function factories - Generate functions with different configurations
3. Event handlers - Retain references to data when handling events
4. Callbacks - Pass context into async operations

Gotchas

• Closures hold a reference to the variable, not the value at creation time
• Be careful with closures in loops - use let instead of var
• Memory leaks can occur if closures hold references to large objects unnecessarily