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feat: Add getPossibleTypesForSelectorClass to Language interface - #148
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Kuldeep2822k
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| ## Help Needed | ||
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| I am willing to submit a pull request with the reference implementation for this RFC once it is accepted. Performance benchmarking (using `npm run test:performance`) will be done to confirm there is no regression. |
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I tested the performance with and without the current switch case for "class" in analyzeParsedSelector using npm run test:performance. There was no difference as no core rule uses the :function selector, so we would need another benchmark for this.
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You're right — no core rule uses :function, so test:performance can't exercise this path. I've dropped that reference from the RFC. The architectural framing (delegating static-analysis ownership to the language plugin) was always the primary motivation here .
| fallback: vk.getKeys, | ||
| matchClass: this.#language.matchesSelectorClass ?? (() => false), | ||
| nodeTypeKey: this.#language.nodeTypeKey, | ||
| + language: this.#language, |
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The other properties and methods from this.#language are passed directly, so why pass the whole language instead of only getPossibleTypesForSelectorClass?
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Good point, you're right. I'll switch to passing only the method.
The reason I had it as the full language was the cache — it's currently a WeakMap keyed by language object. Since the JS language exports a singleton and getSelectorClassNodeTypes is a stable reference, I can key the cache on the method instead and keep the pattern consistent:
js getSelectorClassNodeTypes: this.#language.getSelectorClassNodeTypes,
Pushing an update shortl
| const noLanguageCache = new Map(); | ||
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| function getLanguageCache(language) { | ||
| if (!language) { |
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The language is always set, so there is no need for noLanguageCache.
| - ]; | ||
| - } | ||
| - return null; | ||
| + return language?.getPossibleTypesForSelectorClass?.(selector.name) ?? null; |
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| + return language?.getPossibleTypesForSelectorClass?.(selector.name) ?? null; | |
| + return language.getPossibleTypesForSelectorClass?.(selector.name) ?? null; |
The language cannot be nullish.
nzakas
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nzakas
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Thanks for putting this together. I think it would be simpler to create a property map on each language. See my comment inline.
Question: Did AI write this RFC?
| - **`string[]`** — Only nodes of these types could match the pseudo-class. The traverser will only invoke selector matching for these node types, skipping all others. | ||
| - **`null`** — Any node type could match. The traverser must check every node (this is the safe fallback). | ||
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| **If a language does not implement this method**, the core falls back to returning `null` for all class selectors, which is functionally equivalent to the current behavior for all pseudo-classes _except_ `:function` in JS. Existing language implementations remain unaffected. |
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Just for clarity, you're saying that any unmatched selectors today match all nodes? That's the current behavior?
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I need to clarify — I conflated static analysis with runtime matching. Class selectors other than :function do not match all nodes at runtime. What I meant is that in the static analysis layer (analyzeParsedSelector()), they get nodeTypes: null, which means the traverser can't narrow which node types to check — so it tests them against every node. But esquery.matches() still filters correctly at runtime (e.g., :statement matched 9 out of 36 nodes on a test file, not all 36).
| Instead of a separate method, modify `matchesSelectorClass()` to optionally return type information. This was rejected because it changes the semantics of an existing method and makes the return type complex (boolean vs. type array). | ||
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| ## Open Questions |
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I think another question is whether unknown selectors should actually match all nodes.
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before adding this i want to mention something :- an unknown class name doesn't actually match all nodes, it throws. matchesSelectorClass has a default case that throws "Unknown class name" (lib/languages/js/index.js line 226). so the static analysis says "could match any node", but the runtime throws as soon as the selector's class match is evaluated. if i am wrong do correct me
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| 5. **Type definitions.** The method is added as an optional property (`getSelectorClassNodeTypes?`) in `@eslint/core`'s `Language` interface, so existing TypeScript users are unaffected. | ||
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| ## Alternatives |
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I think another, simpler alternative is to create a selectorClassNodeTypes pubic field on the Language interface that is Map<string, Array<string>>. The core can always convert the class name to lowercase and then match against the map.
I don't think we need to make this a method because there really isn't any additional logic necessary.
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I agree this is a simpler and better approach. Would you like me to revise the RFC to use the Map field as the main proposal, or add it as an alternative?
yes i did use ai to help me draft it |
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@Kuldeep2822k apologies for my late reply. Please update the RFC based on my comments. When I ask for clarification, that means it should go directly into the RFC. When I suggest we use a map instead, please update the RFC to use that as the approach. Thanks. |
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@Kuldeep2822k are you still working on this? |
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@nzakas Yes, still working on this sorry for missing the previous ping |
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@nzakas Done 👍 Switched selectorClassNodeTypes to a Map and added the requested clarifications directly to the RFC. |
| matchesSelectorClass(className, node, ancestry) { | ||
| switch (className.toLowerCase()) { | ||
| case "rule": | ||
| return node.type === "Rule" || node.type === "Atrule"; | ||
| case "at-rule": | ||
| case "atrule": | ||
| return node.type === "Atrule"; | ||
| case "declaration": | ||
| return node.type === "Declaration"; | ||
| default: | ||
| throw new Error(`Unknown class name: ${className}`); | ||
| } | ||
| }, |
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Would it make sense to add a default implementation for matchesSelectorClass() to @eslint/plugin-kit that simply mirrors the mappings in selectorClassNodeTypes like this code currently does? Something like this:
const nodeTypes = this.selectorClassNodeTypes?.get(className.toLowerCase());
if (nodeTypes) {
return nodeTypes.includes(node.type);
}
throw new Error(`Unknown class name: ${className}`);That way, language plugins would be able to use pseudo-class selectors by simply exposing the selectorClassNodeTypes property.
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Yes, I think this would be a good idea so we can avoid every language needing to implement its own method.
We might also want to deprecate matchesSelectorClass altogether and just have core directly use the map.
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A function like matchesSelectorClass allows more specialized matching than simply checking a node's type against a set of known names. For example, in the js language, :expression matches Identifier nodes whose parent is not a MetaProperty (source).
We could argue that this semantic is confusing and that new languages should rely only on node types when defining pseudoclasses. From that perspective, I'd also be mildly in favor of deprecating matchesSelectorClass.
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I have implemented the suggestion but there are some changes i did :-
I've updated the RFC (Section 5) to incorporate @fasttime's suggestion:
- One important adjustment: the default returns
falseon unmapped classes rather than throwing:
When core prunes less aggressively, class selectors can fall to the any-type list and be evaluated on foreign ASTs during multi-language linting (e.g.:functionevaluated against JSON nodes). A throwing default causes runtime crashes on non-JS files, whereas returningfalsealigns with core's existing fallback (matchClass: this.#language.matchesSelectorClass ?? (() => false)) and preserves existing silent no-match semantics. Config-time validation can still catch true typos up front without crashing traversal.
Regarding deprecating/removing matchesSelectorClass in favor of core reading the map directly, empirical testing confirmed two blockers for JavaScript:
- Ancestry-sensitive matching: As @fasttime noted with
:expression, matching depends on node ancestry (e.g., matching anIdentifierunless its parent is aMetaProperty). A static node-type map cannot represent relational/parent constraints. - Open-ended dialect matching:
:statementmatches by suffix (*Statementand*Declaration), allowing it to seamlessly catch dialect nodes like TypeScript'sTSInterfaceDeclaration. A finite name list cannot represent this without brittle hardcoding or breaking dialect rules.
I have verified against ESLint core showed that removing matchesSelectorClass today either silently drops matches on existing rules (such as padding-line-between-statements and no-shadow-restricted-names) or causes mid-lint crashes.
I've documented the full empirical breakdown, crash matrix, and rule-verification results under Open Questions (item 2) and the Cross-Cutting Interaction Matrix in the RFC.
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughThis RFC proposes an optional ChangesSelector class type declaration
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Other Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@designs/2026-selector-class-types/README.md`:
- Around line 150-157: Define how `@eslint/plugin-kit` installs the default
selector-class matcher for languages that provide selectorClassNodeTypes without
matchesSelectorClass(), and ensure the traverser obtains a matcher bound to the
language object so map lookups use the correct receiver. Preserve custom
matchesSelectorClass() implementations as the higher-priority behavior, while
defaulting only when the custom method is absent.
- Around line 153-156: Update the default matcher’s map-backed node check to
read the node property configured by Language.nodeTypeKey instead of node.type,
while preserving the existing class-name lookup and nodeTypes.includes matching
behavior.
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designs/2026-selector-class-types/README.md
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Summary
Add an optional getPossibleTypesForSelectorClass(className) method to the Language interface that allows languages to declare which AST node types could match a given pseudo-class selector (e.g., :function). This removes hardcoded JavaScript-specific logic from the core selector analysis in lib/linter/esquery.js and enables any language plugin to provide the same optimization.
Related Issues
matchesSelectorClass()Summary by CodeRabbit