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# no-unreachable-loop

[Added in v0.7.3](https://github.com/web-infra-dev/rslint/releases/tag/v0.7.3)
## Configuration


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

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

## Rule Details

Disallows a loop whose body can never run a second time. Every path through the
body leaves the loop — by `break`, `return`, or `throw` — so the loop is a
conditional block written as a loop, and the extra iterations it looks like it
performs never happen. That is usually a mistake: a `break` that belongs inside
an `if`, a `return` that should collect results instead of leaving on the first
element, or a condition that was never finished.

A loop iterates again as soon as one path flows back into it, so a `break` or
`return` guarded by an `if` is fine. A loop the surrounding code can never reach
is left alone.

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

```javascript
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
  break;
}

while (foo) {
  doSomething(foo);
  foo = foo.parent;
  return;
}

function find(arr, target) {
  for (const item of arr) {
    return item === target;
  }
}
```

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

```javascript
for (let i = 0; i < arr.length; i++) {
  console.log(arr[i]);
}

while (foo) {
  if (bar) {
    break;
  }
  foo = foo.parent;
}

function find(arr, target) {
  for (const item of arr) {
    if (item === target) {
      return item;
    }
  }
}
```

## Options

This rule accepts an options object with one property:

### `ignore`

An array of loop types to leave unchecked. Each entry is one of
`"WhileStatement"`, `"DoWhileStatement"`, `"ForStatement"`,
`"ForInStatement"`, or `"ForOfStatement"`. It defaults to an empty array.

The following configuration allows the idiom that reads only the first entry
of a collection:

```json
{
  "no-unreachable-loop": [
    "error",
    { "ignore": ["ForInStatement", "ForOfStatement"] }
  ]
}
```

```javascript
function firstKey(obj) {
  for (const key in obj) {
    return key;
  }
  return null;
}
```

## Differences from ESLint

- rslint accepts double-labelled `while` and `do-while` loops when `continue`
  targets the outer label. ESLint 10.8.0 reports those two forms even though
  the jump starts another iteration.
- rslint accepts an enclosing loop when `continue` in a `finally` block
  overrides a pending `return` or `throw`. ESLint 10.8.0 reports some of these
  code paths even though control starts another iteration.

## Original Documentation

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