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docs: Add RuleTester changes to migrate to v9 guide #17923

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merged 2 commits into from Dec 29, 2023
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docs: Add RuleTester changes to migrate to v9 guide #17923

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doc: Add RuleTester changes to migrate to v9 guide
nzakas Dec 29, 2023
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Update docs/src/use/migrate-to-9.0.0.md
nzakas Dec 29, 2023
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12 changes: 12 additions & 0 deletions docs/src/use/migrate-to-9.0.0.md
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Expand Up @@ -29,6 +29,7 @@ The lists below are ordered roughly by the number of users each change is expect
* [Function-style rules are no longer supported](#drop-function-style-rules)
* [`meta.schema` is required for rules with options](#meta-schema-required)
* [`FlatRuleTester` is now `RuleTester`](#flat-rule-tester)
* [Stricter `RuleTester` checks](#stricter-rule-tester)

### Breaking changes for integration developers

Expand Down Expand Up @@ -209,6 +210,17 @@ As announced in our [blog post](/blog/2023/10/flat-config-rollout-plans/), the t

**Related Issues(s):** [#13481](https://github.com/eslint/eslint/issues/13481)

## <a name="stricter-rule-tester"></a> Stricter `RuleTester` checks

In order to aid in the development of high-quality custom rules that are free from common bugs, ESLint v9.0.0 implements several changes to `RuleTester`:

1. **Suggestion messages must be unique.** Because suggestions are typically displayed in an editor as a dropdown list, it's important that no two suggestions have the same message. Otherwise, it's impossible to know what any given suggestion will do. This additional check runs automatically.
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1. **Suggestions must generate valid syntax.** In order for rule suggestions to be helpful, they need to be valid syntax. `RuleTester` now parses the output of suggestions using the same language options as the `code` value and throws an error if parsing fails.

**To address:** Run your rule tests using `RuleTester` and fix any errors that occur. The changes you'll need to make to satisfy `RuleTester` are compatible with ESLint v8.x.

**Related Issues(s):** [#15735](https://github.com/eslint/eslint/issues/15735), [#16908](https://github.com/eslint/eslint/issues/16908)

## <a name="flat-eslint"></a> `FlatESLint` is now `ESLint`

As announced in our [blog post](/blog/2023/10/flat-config-rollout-plans/), the temporary `FlatESLint` class has been renamed to `ESLint`, while the `ESLint` class from v8.x has been renamed to `LegacyESLint`.
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