Closures

Closures

JavaScript closure is a feature that allows inner functions to access the outer scope of a function. Closure helps in binding a function to its outer boundary and is created automatically whenever a function is created. A block is also treated as a scope since ES6. Since JavaScript is event-driven so closures are useful as it helps to maintain the state between events.

Lexical Scoping:

A function scope’s ability to access variables from the parent scope is known as lexical scope. We refer to the parent function’s lexical binding of the child function as “lexically binding.”

Let’s see and understand closure through an example. 

Output:

We can access the variable b which is defined in the function foo() through function inner() as the later preserves the scope chain of the enclosing function at the time of execution of the enclosing function i.e. the inner function knows the value of b through its scope chain. 

This is closure in action that is inner function can have access to the outer function variables as well as all the global variables.

The three outputs are:

1

1

1

Definition

In programming languages, closures (also lexical closures or function closures) are techniques for implementing lexically scoped name binding in languages with first-class functions. Operationally, a closure is a record storing a function[a] together with an environment:[1] a mapping associating each free variable of the function (variables that are used locally, but defined in an enclosing scope) with the value or reference to which the name was bound when the closure was created.

Note: Closure is the concept of function + lexical environment in which function it was created. so every function declared within another function then it has access to the scope chain of the outer function and the variables created within the scope of the outer function will not get destroyed.