no-case-declarations

All the case clauses in a switch share one scope. A let, const or class that you declare in one clause is visible in the others, but it only gets its value if its own clause runs. Reading it from another clause throws a ReferenceError, and declaring the same name in two clauses is a syntax error.

Put braces around the clause to give it its own scope. A function declaration in a clause is reported too. var isn't.

This rule is on by default.

Configure

On by default as a warning. To turn it off:

[lint.rules]
"no-case-declarations" = "off" # or "error"

Examples

Reported:

switch (action.type) {
  case "add":
    const item = action.item;
    list.push(item);
    break;
  default:
    list.length = 0;
}

Not reported:

switch (action.type) {
  case "add": {
    const item = action.item;
    list.push(item);
    break;
  }
  default:
    list.length = 0;
}