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How would you explain JavaScript closures to someone with a knowledge of the concepts they consist of (for example functions, variables and the like), but does not understand closures themselves? ...
I asked a question about currying and closures were mentioned. What is a closure? How does it relate to currying?
I frequently choose to use closures in the Strategy Pattern when the strategy is modified by data at run-time. In a language that allows anonymous block definition -- e.g., Ruby, C# -- closures can be used to implement (what amount to) novel new control structures. The lack of anonymous blocks is among the limitations of closures in Python.
But the callback function in the setTimeout is also a closure; it might be considered "a practical use" since you could access some other local variables from the callback. When I was learning about closures, realising this was useful to me - that closures are everywhere, not just in arcade JavaScript patterns.
3 Closures fit pretty well into an OO world. As an example, consider C# 3.0: It has closures and many other functional aspects, but is still a very object-oriented language. In my experience, the functional aspects of C# tend to stay within the implementation of class members, and not so much as part of the public API my objects end up exposing.
Lambdas and closures are each a subset of all functions, but there is only an intersection between lambdas and closures, where the non-intersecting part of closures would be named functions that are closures and non-intersecting lamdas are self-contained functions with fully-bound variables.
What do the closures capture exactly? Closures in Python use lexical scoping: they remember the name and scope of the closed-over variable where it is created. However, they are still late binding: the name is looked up when the code in the closure is used, not when the closure is created. Since all the functions in your example are created in the same scope and use the same variable name ...
To me a Closure is a (nested?) function with co-located data. When you write software in Haskell and look it through afterwards, you frequently find closures that you have created unintentionally...
With closures the vars referenced are maintained even after the outer function is done or 'closed' if that helps you remember the point. Even with closures, the life cycle of local vars in a function with no inner funcs that reference its locals works the same as it would in a closure-less version.