Closure

A closure is a mechanism that allows a function to "remember" variables from its outer scope even after that outer function has finished executing. The function stores a reference to the environment in which it was created and can access that data later.

What is a Closure

• A closure is formed when an inner function gains access to variables of an outer function.
• When such a function is created, the JS engine stores the lexical environment — the set of variables available at the time of declaration.
• Thanks to this, the inner function continues to have access to these variables even if the outer function is already completed.

Why we use closures

1. Data Encapsulation — creating private variables and protecting state from external interference.
2. State Preservation — preserving values between function calls (e.g., counters, cache).
3. Function Factories — generating functions with individual behavior based on enclosed data.
4. Asynchronous Code — use in callbacks and event handlers to preserve execution context.

Key Ideas

• A closure is the result of lexical binding of functions and environments.
• Variables of the outer scope are not copied, but stored by reference.
• A primary tool for modularity, privacy, and state management.
• Each function call creates its own closure with independent variables.

Links

I never understood JavaScript closures — Medium