ecmascript 6 - How does Javascript declare function parameters?

Question

function func(x = y, y = 2) { 
    return [x, y];
}

func(); // ReferenceError: y is not defined
func(1); // [1, 2]

As the code above implied, there is a hidden TDZ in the function parameters scope, which explains why the code below fails:

function func(arg) {
    let arg = 1; // SyntaxError: Identifier 'arg' has already been declared
}

So function parameters should be declared with let, but what confused me is :

function func(arg) {
    var arg = 1;
    console.log(arg); // 1
}

this code works fine.

Why you can redeclare variables using var? How does Javascript declare function parameters?

---

edit: I know exactly you can't use let to redeclare a variable. The question here is if function parameters is declared using let, so:

function func(arg) {
    var arg = 1;
}

is like:

let arg; // arg parameter declares here
var arg = 1; // func body 

and why this can run without an exception?

Answer 1

there is a hidden TDZ in the function parameters scope

Yes indeed. Have a look here for some more examples.

How does Javascript declare function parameters?

As parameters - see here for a step-by-step explanation. They're neither the same as let nor the same as var, they have their own semantics. ES6, which introduced default initialisers, gave them the same TDZ limitations as in let to catch more programmer mistakes.

Why you can redeclare variables using var?

Because until ES5, redeclaring a variable was not an error condition, and this behaviour needed to be preserved to not break the web. It could only be introduced for new features, like let and const - or argument lists that use default initialisers, try function x(bar, bar) {} vs function x(bar, bar=1){}.