Appreciated closure function

 function fn() {
        return function () { //s;
            console.log("hello")
            return function () { //s1
                console.log("world")
            }
        }
    }
    var s=fn()
    console.log(s);
    var s1=s()
    console.log(s1)
    s1();

The first step: fn assigned to s, console.log (s) output is the return value. 

Step: Because the return value is a function of s, the s () is assigned to s1, var s1 = s () , equivalent to the operation s (), the output hello, and the s is assigned to the S1, 
the console.log ( s1) is a function that returns output S1; 

step: s1 (); performed s1, the output hello