Closure Scope Chain

A closure is the combination of an enclosed structure with references to its surrounding state.A closure from an inner function gives us access to the domain of an external function. Closures are generated in JavaScript any time a function is created, at the time of creation of the feature.

We have three scopes for each closure:

•          Local Scope (Own scope)
•          Outer Functions Scope
•          Global Scope

A common mistake is not to understand that in the case where the outer function is itself a nested function, access to the scope of the outer function requires the outer function enclosing domain, effectively creating a chain of function scopes.

EXAMPLE

// global scope

let a = 10;

function sum(x){

  return function(y){

    return function(z){

      // outer functions scope

      return function(k){

        // local scope

        return x + y + z + k + a;

      }

    }

  }

}

console.log(sum(1)(2)(3)(4)); // log 20

// We can also write without anonymous functions:

// global scope

let a = 10;

function sum(x){

  return function sum2(y){

    return function sum3(z){

      // outer functions scope

      return function sum4(k){

        // local scope

        return x + y + z + k + b;

      }

    }

  }

}

var s = sum(1);

var s1 = s(2);

var s2 = s1(3);

var s3 = s2(4);

console.log(s3) //log 20

In the above example, we have a series of nested functions all having access to the scope of the outer functions. We may claim in this sense that closures have access to all the outer function scopes within which they have been defined.