no-case-declaration

Requires adding a block when a case or default clause contains a let, class or enum declaration.

Rationale

Block scoped variables are not only visible inside the declaring clause but also in all other clauses. But the declared value is not usable in all other clauses as these are part of the temporal dead zone.

The TypeScript compiler does not reliably generate errors for all invalid uses:

• Variables declared with let can be assigned which results in a ReferenceError at runtime.
• class declartions have no restrictions at all which results in a ReferenceError at runtime.
• enum declarations have no restrictions at all. enums are transpiled as function scoped variables. Therefore you may or may not be initialized depending on the order of statement execution.
• var is always function scoped and has no temporal dead zone.
• const is already handled correctly with an error Variable 'xxx' is used before being assigned.

You should always use a block to restrict the scope of the declarations to avoid accidental use in the temporal dead zone.

Examples

:thumbsdown: Examples of incorrect code

switch (v) {
  case 1:
    let foo = v;
    break;
  default:
    class C {}
}

:thumbsup: Examples of correct code

let foo: number;
switch (v) {
  case 1:
    foo = v; // variable declared outside of switch
    break;
  case 2: {
    class C {} // declaraton inside a block
    break;
  }
  case 3:
    for (let v of []) {} // declaration nested in another block scope
  default:
    // the following declarations are always allowed
    const bar = v;
    var baz = v;
    interface I {}
    type T = I;
    function bas() {}
}