# eslint-vitest-rule-tester

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ESLint rule tester with Vitest.

Provides a better testing experience, supports snapshoting, and does not require `globals: true` in Vitest.

> This module requires ESLint v9.0+.

## Who is using?

- [ESLint Stylistic](https://github.com/eslint-stylistic/eslint-stylistic/blob/main/packages/eslint-plugin-js/rules/array-bracket-newline/array-bracket-newline.test.ts)
- [eslint-plugin-command](https://github.com/antfu/eslint-plugin-command/blob/main/src/commands/inline-arrow.test.ts)
- [eslint-plugin-antfu](https://github.com/antfu/eslint-plugin-antfu/blob/main/src/rules/consistent-list-newline.test.ts)

## Install

```bash
npm i -D eslint-vitest-rule-tester
```

## Usage

### Classical Usage

Simliar style to ESLint's `RuleTester` (test cases with implicit test suites)

```ts
import { run, runClassic } from 'eslint-vitest-rule-tester'
import { expect } from 'vitest'

// Classic RuleTester.run style
runClassic('rule-name', rule, {
  valid: [
    // ...
  ],
  invalid: [
    // ...
  ],
}, {
  parserOptions: {
    ecmaVersion: 2020,
    sourceType: 'module',
  },
})

// Or everyting-in-one-object style
run({
  name: 'rule-name',
  rule,
  parserOptions: {
    ecmaVersion: 2020,
    sourceType: 'module',
  },

  valid: [
    // test cases
  ],
  invalid: [
    // test cases
  ],
})
```

<details>
<summary><b>Feature Extensions</b></summary>

#### `output`

`output` field can be a function to do custom assertions. This would also be compatible with snapshot testing.

```ts
import { run, } from 'eslint-vitest-rule-tester'
import { expect } from 'vitest'

run({
  name: 'rule-name',
  rule,
  invalid: [
    {
      code: 'let foo = 1',
      output(output) {
        expect(output.slice(0, 3)).toBe('let')
        expect(output)
          .toMatchInlineSnapshot(`"const foo = 1;"`)
        // Any custom assertion...
      },
    },
  ],
})
```

#### `errors`

`errors` field can be a function to do custom assertion, same as `output`.

```ts
import { run } from 'eslint-vitest-rule-tester'
import { expect } from 'vitest'

run({
  name: 'rule-name',
  rule,
  invalid: [
    {
      code: 'let foo = 1',
      errors(errors) {
        expect(errors).toHaveLength(1)
        expect(errors.map(e => e.messageId))
          .toMatchInlineSnapshot(`["error-message-id"]`)
        // Any custom assertion...
      },
    },
  ],
})
```

#### `onResult` hook

`onResult` field can be a function to do custom assertions with the entire result object.

```ts
import { run } from 'eslint-vitest-rule-tester'
import { expect } from 'vitest'

run({
  name: 'rule-name',
  rule,
  invalid: [
    'let foo = 1',
  ],
  onResult(testCase, result) {
    if (testCase.type === 'invalid')
      expect(result).toMatchSnapshot()
    // here you can't use `toMatchInlineSnapshot` because it's not in the test case
  },
})
```

</details>

### Explicit Test Suites

```ts
import { createRuleTester } from 'eslint-vitest-rule-tester'
import { describe, expect, it } from 'vitest'

describe('rule-name', () => {
  const { valid, invalid } = createRuleTester({
    name: 'rule-name',
    rule,
    configs: {
      // flat config options
      languageOptions: {
        parserOptions: {
          ecmaVersion: 2020,
          sourceType: 'module',
        },
      },
    }
  })

  it('valid case 1', () => {
    valid('const foo = 1')
  })

  it('invalid case 1 with snapshot', () => {
    const result = invalid({
      code: 'const foo = 1',
      errors: ['error-message-id'],
    })

    expect(result.output).toMatchSnapshot()
  })
})
```

## Sponsors

<p align="center">
  <a href="https://cdn.jsdelivr.net/gh/antfu/static/sponsors.svg">
    <img src='https://cdn.jsdelivr.net/gh/antfu/static/sponsors.svg'/>
  </a>
</p>

## License

[MIT](./LICENSE) License © 2024-PRESENT [Anthony Fu](https://github.com/antfu)

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