What is a Closure?

A closure is the combination of a function bundled together with references to its surrounding state (the lexical environment). In other words, a closure gives you access to an outer function's scope from an inner function.

Example

function createCounter() {
  let count = 0;
  return {
    increment: function() {
      count++;
      return count;
    },
    getCount: function() {
      return count;
    }
  };
}

const counter = createCounter();
console.log(counter.getCount()); // 0
counter.increment(); // 1
console.log(counter.getCount()); // 1

Key Points

• Closures are created every time a function is created, at function creation time.
• A closure allows an inner function to access variables from its outer scope even after the outer function has returned.
• Closures are commonly used to create private variables and methods.

Common Interview Follow-up Questions

1. How would you use closures to create private variables?
2. What are the potential memory implications of closures?
3. Can you explain how closures work in loops?

Real-world Applications

• Module Pattern
• Currying
• Memoization
• Maintaining state in async operations