What is closure?
A closure is a function that “remembers” the variables from the surrounding scope where it was created, even after that surrounding function has finished running. In simple terms, it’s a package that contains the function itself plus the data it needs from its original environment.
Let's break it down
- Imagine a function A that defines a variable x and then creates an inner function B that uses x.
- When A runs, it creates B and then finishes.
- Because B is a closure, it carries a hidden reference to x, so later when you call B, it can still read (or even change) x even though A is long gone.
- This works because most programming languages use lexical scoping: a function’s scope is determined by where it is written in the source code, not where it is called.
Why does it matter?
Closures let you keep data private and maintain state without using global variables. They make it easy to write functions that are customized on the fly, create “function factories,” and handle asynchronous code (like callbacks) that need access to values from earlier in the program.
Where is it used?
- Event handlers in web pages (e.g., a button click that needs to know which item was selected).
- Timers and asynchronous APIs (setTimeout, promises) that run later but need earlier values.
- Module patterns that hide internal variables while exposing public methods.
- Functional programming techniques such as map, filter, and reduce where a small helper function remembers a configuration value.
- React hooks (useState, useEffect) rely on closures to keep track of component state between renders.
Good things about it
- Enables data encapsulation and privacy.
- Allows creation of reusable, configurable functions.
- Simplifies handling of asynchronous code by preserving context.
- Reduces the need for global variables, leading to cleaner code.
- Powerful tool for functional programming patterns.
Not-so-good things
- Can unintentionally keep large objects in memory, leading to memory leaks if closures outlive their usefulness.
- Makes debugging harder because variables are hidden inside the closure’s hidden scope.
- Overuse can lead to confusing code, especially for beginners unfamiliar with lexical scoping.
- In some languages, excessive closures may impact performance due to extra memory allocation.