QUESTION: Что такое замыкание (closure)?

A closure in JavaScript is a fundamental concept that is closely related to how the language handles variable scope. The main essence of a closure is that an inner function always remembers its lexical environment. This means that the variables that were located at the place of its creation remain accessible to it, even after the outer function has completely finished its execution.

In practice, closures are actively used to create private variables that cannot be modified directly from the global scope. This approach significantly increases the security and reliability of the developed code. For example, you can write a counter function that encapsulates internal state and provides special methods to read and modify it, completely hiding the variable itself from external interference.

Another popular use case is function factories. These are functions that generate other functions with pre-configured behavior, such as functions for logging messages with a specific prefix. Closures also form the basis of currying — the process of transforming a function with multiple arguments into a sequence of nested functions that take arguments one by one.

Understanding closures is critical for writing efficient JavaScript code. However, careless use of them can lead to unwanted consequences. In particular, if references to large external data structures are kept longer than necessary, memory leaks can occur.