No lexical declarations inside CaseClause StatementList

The following code has bad semantics:

let x = 2;
switch (2) {
  case 1:
    let x = 42;
    break;
  case 2:
    let y = x;  // TDZ error for x since it was never initialized.
   break;
}

because let/const (and class and functions) are scoped to the switch block.

It would be nice to have a lint rule saying that let, const, class and function are not allowed (as direct children) in the StatementList of a CaseClause (and same for DefaultClause).

[arv]

That is a different issue. That was a bad example. Let me try to clarify.

The problem is that lexical declarations are “hoisted” to the nearest block which is the SwitchStatement. The problem is that the CaseClause is not a block in JS. Here is another example:

switch (2) {
  case 1:
    let x = 1;
    print(x);
    break;
  case 2:
    let y = x;  // This always fails.
    print(y);
    break;
 }

The right way to write this is:

switch (2) {
  case 1: {
    let x = 1;
    print(x);
    break;
  }
  case 2: {
    let y = x;  // eslint: x is undefined
    print(y);
    break;
  }
 }

[nzakas]

Thanks for explaining. Go ahead and create a PR.