Let’s see and understand closure through an example.
Explanation: We can access the variable b which is defined in 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 it’s 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.
Definition of Closure:
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: 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.[b]
In other words, closure is created when a child function keep the environment of the parent scope even after the parent function has already executed
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.
Now let’s look at another example.
Explanation: In the above example we used a parameter function rather than a default one. Note even when we are done with the execution of foo(5) we can access the outer_arg variable from the inner function. And on the execution of the inner function produce the summation of outer_arg and inner_arg as desired.
Now let’s see an example of closure within a loop.
In this example we would to store a anonymous function at every index of an array.
4 4 4 4
Explanation: Did you guess the right answer? In the above code we have created four closure which point to the variable i which is local variable to the function outer. Closure don’t remember the value of the variable it only points to the variable or stores the reference of the variable and hence, returns the current value. In the above code when we try to update the value of it gets reflected to all because the closure stores the reference.
Lets see the correct way to write the above code so as to get different values of i at different indexes.
0 1 2 3
Explanation: In the above code we are updating the argument of the function create_Closure with every call. Hence, we get different values of i at different index.
Note : It may be slightly difficult to get the concept of closure at once but try experimenting with closure in different scenarios like for creating getter/setter, callbacks and so on.
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