closure

closure

https://github.com/getify/You-Dont-Know-JS/blob/master/scope%20&%20closures/ch5.md

Closure is when a function is able to remember and access its lexical scope even 
when that function is executing outside its lexical scope.

function foo() {
    var a = 2;

    function bar() {
        console.log( a ); // 2
    }

    bar();
}

foo();

1. lexical scope look-up rules (nested function scope) can kind of explains how function bar() has access to 
    the variable a in the outer enclosing scope.
    
2. but "From a purely academic perspective, what is said of the above snippet is that the function bar()
    has a closure over the scope of foo() (and indeed, even over the rest of the scopes it has access to,
    such as the global scope in our case). Put slightly differently, it's said that bar() closes over the
    scope of foo(). Why? Because bar() appears nested inside of foo(). Plain and simple.