about javascript callback example

a = [3, 1, 2];

function b(v1, v2) {
  console.log('c', v1, v2);
  return 0
};

a.sort(b);

console.log(a);

a result of this script is like this

c 31
c 12
[3, 1, 2]

my question is

1. Why is the function b being called twice?

2. And
   in the first log
   console.log(‘c’, v1, v2);

why the v1 value is the first array value of a. it’s “3”

also why v2 is the second array value of “a”

in the second log
console.log(‘c’, v1, v2);
Why are the 2nd and 3rd values of the array in v1, v2
the result is “c 1 2”

I can’t understand