Scope problems related to the default value of es6- function

Code is extracted from getting started with es6 

var x = 1;
function foo(x, y = function() { x = 2; }) {
  var x = 3;
  y();
  console.log(x);
}

foo() // 3
x // 1
There is a sentence in

.

Another internal variable x is declared inside the
function foo, which is not in the same scope as the first parameter x because it is not in the same scope

then I modified 

var x = 1;
function foo(x, y = function() { x = 2; }) {
  let x = 3;                   //
  y();
  console.log(x);
}

my question is, since it is not the same scope, why can"t we declare x internally with let,
I know that variables cannot be repeatedly declared by let, but the answer says it is not the same scope

my question and guess:
does it not take into account the independent scope generated when the function has default parameters in the process of precompilation, so the duplicate definition is judged first?
feels strange, personal feeling: the compiler should declare the formal parameter x by let, so as to highlight the independent scope

Javascript
Sep.08,2021 
function foo(x, y = function() { x = 2; }) {
  let x = 3;                   //
  y();
  console.log(x);
}

the x declared by let here repeats with the formal parameter, not with var x = 1; repeats 

var x = 1;
function foo(x, y = function() { x = 2; }) {
  var x = 3;
  y();
  console.log(x);
}

foo() // 3
x // 1
Another internal variable x is declared inside the
function foo, which is not in the same scope as the first parameter x because it is not in the same scope

reorganize. What I said before is still wrong. Correct the [[scope]]:
formal parameter declaration form of
foo. I think it is var x; let y = function () {x = 2}

. 
go:x = 1;
1{
    y=function() {x = 2};   //xyx;yx
    2{
       
        var x = 3;       xxy    
    }
}
letx

parameter variables are declared by default, so you cannot declare them again with let or const. 

  
 Parameter  xint2  and  let x = 3  Why are they not in the same scope? 
 es6 parameter default value statement I don't know where to look at the local declaration of let (forget) 
 when I get to the default value, I will go to the let repeated declaration and throw 
 of  error .

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