A closure is the combination of a function and the lexical environment within which that function was declared. Note that Merely accessing a variable outside of the immediate scope (no return statement is necessary) will create something called a closure. A function accessing a global variable will also create a closure as can be seen by console.dir(function).
A closure is a function having access to the parent scope, even after the parent function has closed.
One example of A closure is a function that returns a function. The function that is returned (the inner function) is created inside the called function (the outer) so – due to the scoping rules we’ve seen – the inner has access to the variables and arguments of the outer.
JavaScript variables can belong to the local or global scope. Global variables can be made local (private) with closures.
Mozilla Development Network(MDN) gives a great definition:
“A closure is a special kind of object that combines two things: a function, and the environment in which that function was created. The environment consists of any local variables that were in-scope at the time that the closure was created.”
JavaScript has lexical scoping with function scope. The closure has three scope chains: it has access to its own scope (variables defined between its curly brackets), it has access to the outer function’s variables, and it has access to the global variables.
http://www.htmldog.com/guides/javascript/advanced/closures/
http://howtonode.org/why-use-closure
https://spin.atomicobject.com/2014/10/20/javascript-scope-closures/
Uses:
1) Most useful for events and callbacks
2) Used to simulate private variables i.e, encapsulation. Read more on Private members in javascript and privileged methods here : http://www.crockford.com/javascript/private.html
3) Used to create function factories
https://medium.com/written-in-code/practical-uses-for-closures-c65640ae7304