case or default clauses to be scoped with brackets.

Without brackets in the case or default block, the lexical declarations are visible to the entire switch block but only get initialized when they are assigned, which only happens if that case/default is reached. This can lead to unexpected errors. The solution is to ensure each case or default block is wrapped in brackets to scope limit the declarations.

Invalid:

switch (choice) {
  // `let`, `const`, `function` and `class` are scoped the entire switch statement here
  case 1:
    let a = "choice 1";
    break;
  case 2:
    const b = "choice 2";
    break;
  case 3:
    function f() {
      return "choice 3";
    }
    break;
  default:
    class C {}
}

Valid:

switch (choice) {
  // The following `case` and `default` clauses are wrapped into blocks using brackets
  case 1: {
    let a = "choice 1";
    break;
  }
  case 2: {
    const b = "choice 2";
    break;
  }
  case 3: {
    function f() {
      return "choice 3";
    }
    break;
  }
  default: {
    class C {}
  }
}