trying to figure out how closure works over params and body-vars

1.js

function foo(x,y = function(){ return x; }) {
  console.log( "a:", x );
  var x = 1;
  console.log( "b:", y() );
}

foo(2);

2.js

function bar(x,y = function(){ console.log( "d:", x ); x = 1; }) {
  var x;
  console.log( "c:", x );
  x = 2;
  y();
  console.log( "e:", x );
}

bar(3);

3.txt

Chrome 52-
a:2, b:2, c:3, d:3, e:2

Firefox51-
a:2, b:1, c:3, d:2, e:1

Babel(6/SiteRepl)-
a:2, b:1, c:3, d:2, e:1