Surprising behavior of function parameters default value in JavaScript

Question

```javascript
function native(x, f = () => x) {
var x;
x = 2;
return f();
}

function compiled(x, f) {
if (f === undefined) f = () => x;
var x;
x = 2;
return f();
}

native(1); // returns 1
compiled(1); // returns 2
```
The first function is a simplified version of a quiz question code. I assumed es6 function parameters default values behave just like a check for undefined, which is also basically what babel outputs.
I also assumed function parameters are just like any other local variables defined using ```var```. Which means declaring x multiple times using var in the same function shouldn't make a difference (it gets hoisted to the top of the function and doesn't throw any errors) but surprisingly removing the line ```var x;``` from both functions makes them behave the same. Any idea what's going on?

Answer 1

Nope, maybe go back to previous lessons to see the answer?

Answer 2

What lesson? Did you even read the question?