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# no-fallthrough

[Added in v0.4.2](https://github.com/web-infra-dev/rslint/releases/tag/v0.4.2)
## Configuration

| Preset                   | Configured Value |
| ------------------------ | ---------------- |
| ✅ js.configs.recommended | `"error"`        |


```ts title=rslint.config.ts
import { defineConfig, js } from '@rslint/core';

export default defineConfig([
  js.configs.recommended,
  {
    rules: {
      'no-fallthrough': 'error',
    },
  },
]);
```

## Rule Details

Disallow fallthrough of `case` statements. A case clause that has statements but does not end with a control flow statement (`break`, `return`, `throw`, `continue`) will fall through to the next case, which is usually a programming error. Empty case clauses (with no statements) are allowed by default.

A comment containing "falls through" or "fall through" (case-insensitive) between the current case and the next will suppress the warning.

Examples of **incorrect** code for this rule:

```javascript
switch (foo) {
  case 0:
    a();
  case 1:
    b();
    break;
}

switch (foo) {
  case 0:
    a();
  default:
    b();
}
```

Examples of **correct** code for this rule:

```javascript
switch (foo) {
  case 0:
    a();
    break;
  case 1:
    b();
    break;
}

switch (foo) {
  case 0:
  case 1:
    a();
    break;
}

switch (foo) {
  case 0:
    a();
  /* falls through */
  case 1:
    b();
    break;
}

function bar() {
  switch (foo) {
    case 0:
      a();
      return;
    case 1:
      b();
  }
}
```

## Options

### `commentPattern`

A custom regular expression pattern to match fallthrough comments, applied case-sensitively as `new RegExp(commentPattern, "u")`. By default matches `/falls?\s?through/iu`.

```json
{ "no-fallthrough": ["error", { "commentPattern": "break[\\s\\w]*omitted" }] }
```

### `allowEmptyCase`

When set to `true`, allows case clauses containing only empty statements (`;`) to fall through without a comment.

```json
{ "no-fallthrough": ["error", { "allowEmptyCase": true }] }
```

### `reportUnusedFallthroughComment`

When set to `true`, reports fallthrough comments on cases that cannot actually fall through (e.g., cases ending with `break`).

```json
{ "no-fallthrough": ["error", { "reportUnusedFallthroughComment": true }] }
```

## Original Documentation

- [ESLint: no-fallthrough](https://eslint.org/docs/latest/rules/no-fallthrough)
- [Source code](https://github.com/eslint/eslint/blob/v10.8.1/lib/rules/no-fallthrough.js)