feat(eslint-plugin-mobx): add types for flat/legacy configs

Rewrites the plugin in TypeScript so mobx.flatConfigs.recommended and
mobx.configs.recommended are typed when consumed from a TypeScript
eslint.config.ts, per @kubk's suggestion on the PR:

- source files renamed to .ts
- rollup compiles src/index.ts with rollup-plugin-typescript2 (already
  used by the other packages in this monorepo), with Babel still
  running afterwards (via the existing .babelrc.js) to downlevel the
  output the same way the pre-TypeScript build did
- declarations are generated into dist/index.d.ts and "types" points
  there, so the declaration can no longer drift from the implementation
  the way a hand-written one could
- the generated declaration's `export default` is patched to `export =`
  after the build, to match the actual module.exports = <value> runtime
  shape regardless of the consumer's esModuleInterop setting
- the standalone .d.ts rollup-plugin-typescript2 emits for every
  compiled module (not just the bundled entry point) is removed after
  the build, since dist/index.d.ts is fully self-contained
- @types/eslint moved to dependencies, since the generated declaration
  references it and it isn't installed transitively as a devDependency;
  @types/estree is pinned to ^1.0.0 as a direct dependency too, since
  @types/eslint's own dependency on it is unconstrained and this repo
  already has an incompatible 0.0.39 hoisted at the root

Closes #4545.

Author: rakleed

.changeset/eslint-plugin-mobx-flat-config-types.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-mobx": patch
+---
+
+Add TypeScript types for `configs.recommended` and `flatConfigs.recommended`, so they're typed when used from a TypeScript `eslint.config.ts`.

package-lock.json

@@ -1,6 +1,6 @@
 import { getRuleTester } from "./utils/get-rule-tester";
 
-import rule from "../src/exhaustive-make-observable.js";
+import rule from "../src/exhaustive-make-observable";
 
 const tester = getRuleTester();
 

packages/eslint-plugin-mobx/__tests__/exhaustive-make-observable.js

@@ -1,6 +1,6 @@
 import { getRuleTester } from "./utils/get-rule-tester";
 
-import rule from "../src/missing-make-observable.js";
+import rule from "../src/missing-make-observable";
 
 const tester = getRuleTester();
 

packages/eslint-plugin-mobx/__tests__/missing-make-observable.js

@@ -1,6 +1,6 @@
 import { getRuleTester } from "./utils/get-rule-tester";
 
-import rule from "../src/missing-observer.js"
+import rule from "../src/missing-observer"
 
 const tester = getRuleTester();
 

packages/eslint-plugin-mobx/__tests__/missing-observer.js

@@ -1,6 +1,6 @@
 import { getRuleTester } from "./utils/get-rule-tester";
 
-import rule from "../src/no-anonymous-observer.js"
+import rule from "../src/no-anonymous-observer"
 
 const tester = getRuleTester();
 

packages/eslint-plugin-mobx/__tests__/no-anonymous-observer.js

@@ -1,6 +1,6 @@
 import { getRuleTester } from "./utils/get-rule-tester";
 
-import rule from "../src/unconditional-make-observable.js";
+import rule from "../src/unconditional-make-observable";
 
 const tester = getRuleTester();
 

packages/eslint-plugin-mobx/__tests__/unconditional-make-observable.js

@@ -3,6 +3,7 @@
     "version": "0.0.14",
     "description": "ESLint rules for MobX",
     "main": "dist/index.js",
+    "types": "dist/index.d.ts",
     "repository": {
         "type": "git",
         "url": "https://github.com/mobxjs/mobx.git",
@@ -27,6 +28,10 @@
     "peerDependencies": {
         "eslint": "^3.0.0 || ^4.0.0 || ^5.0.0 || ^6.0.0 || ^7.0.0 || ^8.0.0 || ^9.0.0 || ^10.0.0"
     },
+    "dependencies": {
+        "@types/eslint": "^9.6.1",
+        "@types/estree": "^1.0.0"
+    },
     "devDependencies": {
         "eslint-7": "npm:eslint@^7.0.0",
         "eslint-9": "npm:eslint@^9.0.0"
@@ -41,6 +46,8 @@
         "test:7": "jest --config jest.config-eslint-7.js",
         "test:9": "jest --config jest.config-eslint-9.js",
         "test": "npm run test:7 && npm run test:9",
+        "test:types": "tsc --noEmit",
+        "test:check": "npm run test:types",
         "build": "rollup --config",
         "prepublishOnly": "npm run build"
     }

packages/eslint-plugin-mobx/package.json

@@ -1,21 +1,73 @@
+import fs from "fs";
+import path from "path";
 import { nodeResolve } from "@rollup/plugin-node-resolve";
 import commonjs from "@rollup/plugin-commonjs";
 import babel from "@rollup/plugin-babel";
+import typescript from "rollup-plugin-typescript2";
 import pkg from "./package.json";
 
+// rollup-plugin-typescript2 only strips types and downlevels down to the
+// root tsconfig's `target` (es6), so arrow functions, `const`/`let`,
+// destructuring, template literals etc. would otherwise reach dist/index.js
+// as-is. Babel still runs afterwards (via .babelrc.js, same as before the
+// TypeScript migration) to downlevel that to the postinstall-tested `eslint`
+// peer range down to ^3.0.0.
+//
+// rollup-plugin-typescript2 also writes a standalone .d.ts for every source
+// module it compiles (dist/utils.d.ts, dist/missing-observer.d.ts, ...), not
+// just the bundled entry point — dist/index.d.ts is fully self-contained and
+// none of them are referenced from anywhere, so they're removed below.
+//
+// Finally, rollup-plugin-typescript2 requires an ES module target so Rollup
+// can bundle the sources, so src/index.ts is written as `export default
+// {...}` — but the bundled runtime output is a single CommonJS export,
+// `module.exports = <value>`, not an ES default export wrapped in `.default`.
+// A declaration using `export default` only lines up with that shape under
+// `esModuleInterop`; a plain `import mobx from "eslint-plugin-mobx"` without
+// it would type-check but read `mobx.default`, which is `undefined` at
+// runtime. `export =` matches `module.exports = <value>` unconditionally, so
+// the generated declaration is patched to use it too.
+const fixDeclarationOutput = () => ({
+  name: "fix-declaration-output",
+  writeBundle(options) {
+    const distDir = path.dirname(options.file);
+    const declarationPath = path.join(distDir, "index.d.ts");
+    const declaration = fs.readFileSync(declarationPath, "utf8");
+    const patched = declaration.replace(/^export default (\w+);\s*$/m, "export = $1;");
+    if (patched === declaration) {
+      throw new Error(
+        `Could not find an "export default <name>;" statement to patch in ${declarationPath}`
+      );
+    }
+    fs.writeFileSync(declarationPath, patched);
+
+    for (const entry of fs.readdirSync(distDir)) {
+      if (entry.endsWith(".d.ts") && entry !== "index.d.ts") {
+        fs.unlinkSync(path.join(distDir, entry));
+      }
+    }
+  },
+});
+
 export default [
   {
-    input: "src/index.js",
+    input: "src/index.ts",
     plugins: [
       nodeResolve(),
       commonjs(),
+      typescript(),
       babel({
         babelHelpers: "bundled",
         exclude: "**/node_modules/**",
+        // @rollup/plugin-babel's default extensions don't include ".ts" - by
+        // the time this runs, rollup-plugin-typescript2 has already emitted
+        // plain JS for these modules, but the module ids still end in ".ts".
+        extensions: [".js", ".jsx", ".es6", ".es", ".mjs", ".cjs", ".ts"],
       }),
+      fixDeclarationOutput(),
     ],
     output: [
       { file: pkg.main, format: "cjs", exports: "auto" },
     ],
   },
-];
+];

packages/eslint-plugin-mobx/rollup.config.js

@@ -1,6 +1,6 @@
-"use strict"
+import type { Rule } from "eslint"
 
-const { findAncestor, isMobxDecorator } = require("./utils.js")
+import { findAncestor, isMobxDecorator } from "./utils"
 
 // TODO support this.foo = 5; in constructor
 // TODO? report on field as well
@@ -42,7 +42,7 @@ function create(context) {
             }
 
             const annotationProps = secondArg?.properties || []
-            const nonAnnotatedMembers = []
+            const nonAnnotatedMembers: any[] = []
             let hasAnyDecorator = false
 
             members.forEach(member => {
@@ -102,7 +102,7 @@ function create(context) {
     }
 }
 
-module.exports = {
+const rule: Rule.RuleModule = {
     meta: {
         type: "suggestion",
         fixable: "code",
@@ -119,8 +119,7 @@ module.exports = {
         ],
         docs: {
             description: "enforce all fields being listen in `makeObservable`",
-            recommended: true,
-            suggestion: false
+            recommended: true
         },
         messages: {
             missingAnnotation:
@@ -129,3 +128,5 @@ module.exports = {
     },
     create
 }
+
+export default rule