concept closure in category functional programming

This is an excerpt from Manning's book Functional Programming in C++. Login to get full access to this book.

In addition to creating a class, evaluating the lambda expression also creates an instance of that class called a closure: an object containing some state or environment along with code that should be executed on that state.

This is an excerpt from Manning's book Functional Programming in JavaScript. Login to get full access to this book.

const princetonZip = zipCode('08544', '3345');
princetonZip.code(); //-> '08544'

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This is a bit mind-bending, and it’s all thanks to the closure that forms around object and function declarations in JavaScript. Being able to access data this way has many practical uses; in this section, we’ll look at using closures to emulate private variables, fetch data from the server, and force block-scoped variables.

A closure is a data structure that binds a function to its environment at the moment it’s declared. It’s based on the textual location of the function declaration; therefore, a closure is also called a static or lexical scope surrounding the function definition. Because it gives functions access to its surrounding state, it makes code clear and readable. As you’ll see shortly, closures are instrumental not only in functional programs when you’re working with higher-order functions, but also for event-handling and callbacks, emulating private variables, and mitigating some of JavaScript’s pitfalls.

Unlike JavaScript, many languages provide a built-in mechanism to define internal properties of an object by setting accessibility modifiers (like private). JavaScript doesn’t have a native keyword for private variables and functions to be accessed only in the scope of an object. Encapsulation can play in favor of immutability because you can’t change what you can’t access.

Using closures, however, it’s possible to emulate this behavior. One example is returning an object, much like zipCode and coordinate in the earlier example. These functions return object literals with methods that have access to any of the outer function’s local variables, but don’t expose these variables, therefore effectively making them private.

Figure 8.4. Passing the current student object as a parameter properly sets the function’s closure and solves the ambiguity resulting from executing remote calls in a loop.