Please explain Heap's algorithm step by step in this recursive javascript code.

Question

function permuteAll(str) {
var arr = str.split('');
var n = arr.length;
var result = [];

perm(arr,n);
return result;

function perm(s,length) {

if( length === 1) {
result.push(s.join(''));
}
for(var i=0;i<length; i++) {
perm(s,length-1);
if(length%2 !== 0) {
swap(s,0,length-1);
}
else if(length%2 === 0) {
swap(s,i,length-1);
}
}
}

function swap(array,x,y){
var tmp = array[x];
array[x] =array[y];
array[y] = tmp;
}
}

console.log(permuteAll('123'));

Can someone please explain to me step by step how does this unbelievable piece of code can output all permutations of the input string.
I already know what recursion is but only on the basic level. Like the basic factorial recursive solution.
I can't seem to grasp how the push call will be called n! times.
I already read the Wikipedia article, stackoverflow threads but still don't get it.
Thanks.

Answer 1

http://ruslanledesma.com/2016/06/17/why-does-heap-work.html