scope calling functions inside functions

Question

Hope somebody finds the time to explain little about functions in functions and scoping. I am trying to understand little more on functions and scope of variables and found a qu

Answer 1

The function sum returns a function, which we refer to as f.

The function f also returns a function: in fact, the function f returns itself.

When the function f is defined inside of sum, it gets permanent access to all variables currently visible in the scope chain. Here, it includes the locally-defined variable sum (the local running sum tally) and f (the function itself). (A "closure" is what we call the functional code of f along with all its in-scope variables.)

Because f returns itself, you can chain f with repeated calls:

var this_is_f = sum(1);
var same_f_again = this_is_f(2);
var f_a_third_time = same_f_again(3);

Or simply:

sum(1)(2)(3);

It is important to note that in my first example, I don't create new functions; instead, I just refer to the exact same function object with three different identifiers.

Each call to sum creates a brand-new f with a new local sum in its scope (here, I mean the local sum defined on the first line of the function named sum). However, calling the function sum does not clobber any old f, because each call to sum instantiates a new f (and knows nothing about any other fs that have been created on prior calls to sum). That way, you can have multiple tallies running:

var first_tally = sum(1)(2);   // first: 3
var second tally = sum(4)(5);  // second: 9
first_tally(3);   // first: 6
second_tally(6);  // second: 15

The reason you're able to see a meaningful result at any time is that f stingifies to the value of sum, instead of showing you its source code.