Question

.

var a,b,c = function() { return 5; };

Variables a and b is undefined, c is function, why when we do this (a,b,c)() we have 5 ?

Answer 1

The declaration is the same as:

var a;
var b;
var c = function() { return 5; };

or practically the same as:

var a;
var b;
function c() { return 5; };

Using (a,b,c) has nothing to do with the declaration, it simply returns the last operand, so (a,b,c)() is exactly the same as c() (as long as evaluating a and b doesn’t have any side effects).