If re-declaration with var will have effect on existed variable

Answer 1

Why the results of show2 and show3 are different.

let's evaluate your code by this way

const show2 = function(x, y = () => {x.value = 2; return x;}) {
    x = {name: "from argument", value: 3};
    console.log(y());//{name: "from argument", value: 2}
    console.log(x);//{name: "from argument", value: 2}
};
show2();

const show3 = function(x, y = () => {if(!x){x = {name:"from function", value: -1}}x.value = 2; return x;}) {
    var x = {name: "from var", value: 3};
    console.log(y());//{name: "from function", value: 2}
    console.log(x);//{name: "from var", value: 3}
};
show3();

const show4 = function(x, y = () => {if(!x){x = {name:"from function", value: -1}}x.value = 2; return x;}) {
    var x = {name: "from var", value: 3};
    console.log(y());//{name: "from outside", value: 2}
    console.log(x);//{name: "from var", value: 3}
};
show4({name:"from outside", value: -1})