A closure is a function that remembers its outer scope even after the outer function has returned. It allows functions to access variables from their parent scope, enabling powerful patterns like function factories and private variables.

In JavaScript, a closure is created when a function is defined inside another function, and the inner function retains access to the variables in the outer function’s scope even after the outer function has finished executing. This behavior is a result of how JavaScript handles scope and functions. When you define a function, it forms a 'closure' around the variables in its scope, meaning it keeps a reference to these variables even if the surrounding context has been destroyed. Closures are often used to create private variables, where a function returns another function that can access variables from its parent scope, but those variables are not accessible from the outside. For example, consider a function createCounter() that returns an inner function that increments a private variable count. Even after createCounter() has executed, the inner function retains access to count. This allows the counter to be incremented independently of any external code. Closures are also used in event handlers and asynchronous programming, where functions need to maintain access to their scope over time. They enable patterns like function factories, where a function generates other functions that have customized behavior based on their scope. However, closures can also lead to memory issues if not used carefully, as variables in the closure remain in memory as long as the closure exists. Understanding closures is essential for writing effective JavaScript, as they form the foundation of many advanced patterns in the language, from callbacks to function currying and beyond.