test(linter/plugins): add types test for VisitorObject

apps/oxlint/package.json

@@ -39,6 +39,7 @@
     "@arethetypeswrong/core": "catalog:",
     "@babel/code-frame": "^7.28.6",
     "@napi-rs/cli": "catalog:",
+    "@type-challenges/utils": "^0.1.1",
     "@types/babel__code-frame": "^7.27.0",
     "@types/esquery": "^1.5.4",
     "@types/estree": "^1.0.8",

apps/oxlint/src-js/visitor.test-d.ts

@@ -0,0 +1,68 @@
+import type { ExpectExtends, ExpectTrue, ExpectFalse } from "@type-challenges/utils";
+import type { VisitorObject } from "./generated/visitor.d.ts";
+import type { Node, CallExpression } from "./generated/types.d.ts";
+
+// Empty visitor object is allowed
+const emptyVisitor = {};
+export type _Empty = ExpectTrue<ExpectExtends<VisitorObject, typeof emptyVisitor>>;
+
+// Specific node visitors have a stricter type for the parameter
+const callExpressionVisitor = {
+  CallExpression: (_node: CallExpression) => {},
+};
+export type _CallExpr = ExpectTrue<ExpectExtends<VisitorObject, typeof callExpressionVisitor>>;
+
+const callExpressionExitVisitor = {
+  "CallExpression:exit": (_node: CallExpression) => {},
+};
+export type _CallExprExit = ExpectTrue<
+  ExpectExtends<VisitorObject, typeof callExpressionExitVisitor>
+>;
+
+const debuggerStatementWrongTypeVisitor = {
+  DebuggerStatement: (_node: CallExpression) => {},
+};
+export type _DebuggerStmtWrongType = ExpectFalse<
+  ExpectExtends<VisitorObject, typeof debuggerStatementWrongTypeVisitor>
+>;
+
+// Node selectors have generic `Node` type for the parameter
+const selectorsVisitor = {
+  "FunctionExpression > Identifier": (_node: Node) => {},
+  ":matches(FunctionExpression, FunctionDeclaration)": (_node: Node) => {},
+};
+export type _Selectors = ExpectTrue<ExpectExtends<VisitorObject, typeof selectorsVisitor>>;
+
+// Visitor functions can omit the node parameter
+const callExpressionNoParamVisitor = {
+  CallExpression: () => {},
+};
+export type _CallExprNoParam = ExpectTrue<
+  ExpectExtends<VisitorObject, typeof callExpressionNoParamVisitor>
+>;
+
+// Properties of visitor object can be `undefined`.
+// Ideally we'd disallow this, but it's not possible without `exactOptionalPropertyTypes: true` in `tsconfig.json`.
+// Obviously we can't force that on users.
+const callExpressionUndefinedVisitor = {
+  CallExpression: undefined,
+};
+export type _CallExprUndefined = ExpectTrue<
+  ExpectExtends<VisitorObject, typeof callExpressionUndefinedVisitor>
+>;
+
+// Other types are not allowed
+const invalidNullVisitor = {
+  CallExpression: null,
+};
+export type _InvalidNull = ExpectFalse<ExpectExtends<VisitorObject, typeof invalidNullVisitor>>;
+
+const invalidObjectVisitor = {
+  CallExpression: {},
+};
+export type _InvalidObject = ExpectFalse<ExpectExtends<VisitorObject, typeof invalidObjectVisitor>>;
+
+const invalidStringVisitor = {
+  CallExpression: "oh dear",
+};
+export type _InvalidString = ExpectFalse<ExpectExtends<VisitorObject, typeof invalidStringVisitor>>;