Closures: The Real Mental Model

A closure is created when a function retains access to variables from its lexical environment even after the outer function has finished executing. The important concept is not merely 'a function inside another function'; it is that the inner function retains access to the relevant lexical bindings.

Closures are used for private state, factories, memoization, callbacks, module patterns, and asynchronous code.

Private State with a Closure

function createCounter() {
  let count = 0;

  return function increment() {
    count += 1;
    return count;
  };
}

const counter = createCounter();
console.log(counter());
console.log(counter());

💡 createCounter returns after creating the inner function, but the returned function still has access to count. The binding remains reachable through the closure.

Key Takeaways

Closures preserve access to lexical bindings.
Closures are useful for encapsulation and state.
A closure retains reachable bindings, not necessarily every variable from the entire outer scope.

Knowledge Check

Why does `count` remain available after `createCounter()` returns?

Closures: The Real Mental Model

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