Well, in switch, if you are declaring it without bracket, that equals to declaring it into your function, which then creates an error of "Identifies "..." has already been declared".

Because doing this:

```

function () {

switch (...) {

case "1":

let a = 1;

// ....

break;

}

}

```

Is technically almost the same as the following:

```

function () {

let a = 1;

}

```

While it may seems normal, The problem will happen with a code that similar like the following:

```

function () {

switch (...) {

case "1":

let a = 1;

// ....

break;

case "2":

let a = 2;

// ....

break;

}

}

```

You will then get the error `Uncaught SyntaxError: Identifier 'a' has already been declared`.

To fix it, simply put bracket.