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noSwitchDeclarations (since v1.0.0)

Diagnostic Category: lint/correctness/noSwitchDeclarations

Sources:

Disallow lexical declarations in switch clauses.

Lexical declarations in switch clauses are accessible in the entire switch. However, it only gets initialized when it is assigned, which will only happen if the switch clause where it is defined is reached.

To ensure that the lexical declarations only apply to the current switch clause wrap your declarations in a block.

Examples

Section titled Examples

Invalid

Section titled Invalid
switch (foo) {
    case 0:
        const x = 1;
        break;
    case 2:
        x; // `x` can be used while it is not initialized
        break;
}
correctness/noSwitchDeclarations.js:3:9 lint/correctness/noSwitchDeclarations  FIXABLE  ━━━━━━━━━━━━

  ✖ Other switch clauses can erroneously access this declaration.
    Wrap the declaration in a block to restrict its access to the switch clause.
  
    1 │ switch (foo) {
    2 │     case 0:
  > 3 │         const x = 1;
      │         ^^^^^^^^^^^
    4 │         break;
    5 │     case 2:
  
  ℹ The declaration is defined in this switch clause:
  
    1 │ switch (foo) {
  > 2 │     case 0:
      │     ^^^^^^^
  > 3 │         const x = 1;
  > 4 │         break;
      │         ^^^^^^
    5 │     case 2:
    6 │         x; // `x` can be used while it is not initialized
  
  ℹ Unsafe fix: Wrap the declaration in a block.
  
    1  1 │   switch (foo) {
    2    │ - ····case·0:
       2 │ + ····case·0:·{
    3  3 │           const x = 1;
    4    │ - ········break;
       4 │ + ········break;
       5 │ + ····}
    5  6 │       case 2:
    6  7 │           x; // `x` can be used while it is not initialized
  

switch (foo) {
    case 0:
        function f() {}
        break;
    case 2:
        f(); // `f` can be called here
        break;
}
correctness/noSwitchDeclarations.js:3:9 lint/correctness/noSwitchDeclarations  FIXABLE  ━━━━━━━━━━━━

  ✖ Other switch clauses can erroneously access this declaration.
    Wrap the declaration in a block to restrict its access to the switch clause.
  
    1 │ switch (foo) {
    2 │     case 0:
  > 3 │         function f() {}
      │         ^^^^^^^^^^^^^^^
    4 │         break;
    5 │     case 2:
  
  ℹ The declaration is defined in this switch clause:
  
    1 │ switch (foo) {
  > 2 │     case 0:
      │     ^^^^^^^
  > 3 │         function f() {}
  > 4 │         break;
      │         ^^^^^^
    5 │     case 2:
    6 │         f(); // `f` can be called here
  
  ℹ Unsafe fix: Wrap the declaration in a block.
  
    1  1 │   switch (foo) {
    2    │ - ····case·0:
       2 │ + ····case·0:·{
    3  3 │           function f() {}
    4    │ - ········break;
       4 │ + ········break;
       5 │ + ····}
    5  6 │       case 2:
    6  7 │           f(); // `f` can be called here
  

switch (foo) {
    case 0:
        class A {}
        break;
    default:
        new A(); // `A` can be instantiated here
        break;
}
correctness/noSwitchDeclarations.js:3:9 lint/correctness/noSwitchDeclarations  FIXABLE  ━━━━━━━━━━━━

  ✖ Other switch clauses can erroneously access this declaration.
    Wrap the declaration in a block to restrict its access to the switch clause.
  
    1 │ switch (foo) {
    2 │     case 0:
  > 3 │         class A {}
      │         ^^^^^^^^^^
    4 │         break;
    5 │     default:
  
  ℹ The declaration is defined in this switch clause:
  
    1 │ switch (foo) {
  > 2 │     case 0:
      │     ^^^^^^^
  > 3 │         class A {}
  > 4 │         break;
      │         ^^^^^^
    5 │     default:
    6 │         new A(); // `A` can be instantiated here
  
  ℹ Unsafe fix: Wrap the declaration in a block.
  
    1  1 │   switch (foo) {
    2    │ - ····case·0:
       2 │ + ····case·0:·{
    3  3 │           class A {}
    4    │ - ········break;
       4 │ + ········break;
       5 │ + ····}
    5  6 │       default:
    6  7 │           new A(); // `A` can be instantiated here

Valid

Section titled Valid
switch (foo) {
    case 0: {
        const x = 1;
        break;
    }
    case 1:
        // `x` is not visible here
        break;
}
Section titled Related links