diff --git a/package-lock.json b/package-lock.json
index 39cc887d3..19f0b29b6 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -95,7 +95,6 @@
         "remark-squeeze-paragraphs": "^5.0.0",
         "remark-strip-badges": "^6.0.0",
         "remark-toc": "^8.0.0",
-        "rimraf": "^3.0.0",
         "rodemirror": "^2.0.0",
         "type-coverage": "^2.0.0",
         "typescript": "^4.0.0",
diff --git a/package.json b/package.json
index bb798eb05..d934dd588 100644
--- a/package.json
+++ b/package.json
@@ -98,7 +98,6 @@
     "remark-squeeze-paragraphs": "^5.0.0",
     "remark-strip-badges": "^6.0.0",
     "remark-toc": "^8.0.0",
-    "rimraf": "^3.0.0",
     "rodemirror": "^2.0.0",
     "type-coverage": "^2.0.0",
     "typescript": "^4.0.0",
@@ -119,7 +118,6 @@
   "scripts": {
     "postinstall": "patch-package",
     "prepare": "husky install",
-    "clean": "npm exec -c \"rimraf node_modules\" --workspaces",
     "docs-prep": "node --unhandled-rejections=strict website/prep.js && postcss docs/_asset/index.css -o public/index.css",
     "docs-bundle-dev": "cross-env NODE_ENV=development node --unhandled-rejections=strict website/bundle.js",
     "docs-bundle-prod": "cross-env NODE_ENV=production node --unhandled-rejections=strict website/bundle.js",
diff --git a/packages/esbuild/lib/index.js b/packages/esbuild/lib/index.js
index dadec2238..d14b25131 100644
--- a/packages/esbuild/lib/index.js
+++ b/packages/esbuild/lib/index.js
@@ -7,10 +7,12 @@
  * @typedef {import('esbuild').Message} Message
  * @typedef {import('vfile').VFileValue} VFileValue
  * @typedef {import('vfile-message').VFileMessage} VFileMessage
- * @typedef {import('unist').Point} Point
  * @typedef {import('@mdx-js/mdx/lib/core.js').ProcessorOptions} ProcessorOptions
- *
- * @typedef {ProcessorOptions & {allowDangerousRemoteMdx?: boolean}} Options
+ */
+
+/**
+ * @typedef {ProcessorOptions & {allowDangerousRemoteMdx?: boolean | null | undefined}} Options
+ *   Configuration.
  */
 
 import assert from 'node:assert'
@@ -32,11 +34,11 @@ const p = process
 /**
  * Compile MDX w/ esbuild.
  *
- * @param {Options} [options]
+ * @param {Options | null | undefined} [options]
  * @return {Plugin}
  */
-export function esbuild(options = {}) {
-  const {allowDangerousRemoteMdx, ...rest} = options
+export function esbuild(options) {
+  const {allowDangerousRemoteMdx, ...rest} = options || {}
   const name = '@mdx-js/esbuild'
   const remoteNamespace = name + '-remote'
   const {extnames, process} = createFormatAwareProcessors(rest)
@@ -123,22 +125,24 @@ export function esbuild(options = {}) {
     }
 
     /**
-     * @param {Omit<OnLoadArgs, 'pluginData'> & {pluginData?: {contents?: string|Buffer}}} data
+     * @param {Omit<OnLoadArgs, 'pluginData'> & {pluginData?: {contents?: Buffer | string | null | undefined}}} data
      * @returns {Promise<OnLoadResult>}
      */
     async function onload(data) {
       /** @type {string} */
       const doc = String(
-        data.pluginData && data.pluginData.contents !== undefined
+        data.pluginData &&
+          data.pluginData.contents !== null &&
+          data.pluginData.contents !== undefined
           ? data.pluginData.contents
           : /* eslint-disable-next-line security/detect-non-literal-fs-filename */
             await fs.readFile(data.path)
       )
 
       let file = new VFile({value: doc, path: data.path})
-      /** @type {VFileValue|undefined} */
+      /** @type {VFileValue | undefined} */
       let value
-      /** @type {Array<VFileMessage|Error>} */
+      /** @type {Array<Error | VFileMessage>} */
       let messages = []
       /** @type {Array<Message>} */
       const errors = []
@@ -150,7 +154,7 @@ export function esbuild(options = {}) {
         value = file.value
         messages = file.messages
       } catch (error_) {
-        const error = /** @type {VFileMessage|Error} */ (error_)
+        const error = /** @type {Error | VFileMessage} */ (error_)
         if ('fatal' in error) error.fatal = true
         messages.push(error)
       }
@@ -225,7 +229,7 @@ export function esbuild(options = {}) {
       // V8 on Erbium.
       /* c8 ignore next 9 */
       return {
-        contents: value,
+        contents: value || '',
         errors,
         warnings,
         resolveDir: http.test(file.path)
diff --git a/packages/esbuild/package.json b/packages/esbuild/package.json
index 7bf05294e..b6b59b7bd 100644
--- a/packages/esbuild/package.json
+++ b/packages/esbuild/package.json
@@ -56,7 +56,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/esbuild/test/index.test.js b/packages/esbuild/test/index.test.js
index 99e96d4ce..3099773c6 100644
--- a/packages/esbuild/test/index.test.js
+++ b/packages/esbuild/test/index.test.js
@@ -1,11 +1,10 @@
 /**
  * @typedef {import('esbuild').BuildFailure} BuildFailure
- * @typedef {import('esbuild').Message} Message
  * @typedef {import('hast').Root} Root
  * @typedef {import('vfile').VFile} VFile
  * @typedef {import('mdx/types.js').MDXContent} MDXContent
  *
- * @typedef {import('remark-mdx')}
+ * @typedef {import('remark-mdx')} DoNotTouchIncludeMathInTree
  */
 
 import {promises as fs} from 'fs'
@@ -251,7 +250,6 @@ test('@mdx-js/esbuild', async () => {
                 assert.ok(text && text.type === 'text')
                 const jsx = head.children[1] // JSX in heading
                 assert.ok(jsx && jsx.type === 'mdxJsxTextElement')
-                console.log(head)
                 file.message('1')
                 file.message('2', eol)
                 file.message('3', tree)
diff --git a/packages/esbuild/tsconfig.json b/packages/esbuild/tsconfig.json
index 94d319f4f..6cac7d0aa 100644
--- a/packages/esbuild/tsconfig.json
+++ b/packages/esbuild/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "index.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/loader/index.cjs b/packages/loader/index.cjs
index 0acb30bfa..60ad1a5cc 100644
--- a/packages/loader/index.cjs
+++ b/packages/loader/index.cjs
@@ -1,3 +1,7 @@
+/**
+ * @typedef {import('webpack').LoaderContext<unknown>} LoaderContext
+ */
+
 'use strict'
 
 /**
@@ -5,6 +9,7 @@
  *
  * @todo once webpack supports ESM loaders, remove this wrapper.
  *
+ * @this {LoaderContext}
  * @param {string} code
  */
 module.exports = function (code) {
diff --git a/packages/loader/index.d.cts b/packages/loader/index.d.cts
new file mode 100644
index 000000000..34362abf8
--- /dev/null
+++ b/packages/loader/index.d.cts
@@ -0,0 +1,7 @@
+// Some TS versions use this file, some `index.d.ts`.
+type LoaderContext = import('webpack').LoaderContext<unknown>
+
+declare function mdxLoader(this: LoaderContext, code: string): void
+export = mdxLoader
+
+export type Options = import('@mdx-js/mdx/lib/core.js').ProcessorOptions
diff --git a/packages/loader/index.d.ts b/packages/loader/index.d.ts
index c9d05356e..ee1fe0ee9 100644
--- a/packages/loader/index.d.ts
+++ b/packages/loader/index.d.ts
@@ -1,3 +1,4 @@
+// Some TS versions use this file, some `index.d.cts`.
 import type {ProcessorOptions} from '@mdx-js/mdx/lib/core.js'
 import type {LoaderContext} from 'webpack'
 
diff --git a/packages/loader/lib/index.js b/packages/loader/lib/index.js
index eedc24887..9869d7bfe 100644
--- a/packages/loader/lib/index.js
+++ b/packages/loader/lib/index.js
@@ -1,13 +1,23 @@
 /**
+ * @typedef {import('@mdx-js/mdx').CompileOptions} CompileOptions
  * @typedef {import('vfile').VFileCompatible} VFileCompatible
  * @typedef {import('vfile').VFile} VFile
  * @typedef {import('vfile-message').VFileMessage} VFileMessage
- * @typedef {import('@mdx-js/mdx').CompileOptions} CompileOptions
- * @typedef {Pick<CompileOptions, 'SourceMapGenerator'>} Defaults
- * @typedef {Omit<CompileOptions, 'SourceMapGenerator'>} Options
  * @typedef {import('webpack').LoaderContext<unknown>} LoaderContext
  * @typedef {import('webpack').Compiler} WebpackCompiler
- * @typedef {(vfileCompatible: VFileCompatible) => Promise<VFile>} Process
+ */
+
+/**
+ * @typedef {Pick<CompileOptions, 'SourceMapGenerator'>} Defaults
+ * @typedef {Omit<CompileOptions, 'SourceMapGenerator'>} Options
+ *   Configuration.
+ *
+ * @callback Process
+ *   Process.
+ * @param {VFileCompatible} vfileCompatible
+ *   Input.
+ * @returns {Promise<VFile>}
+ *   File.
  */
 
 import {createHash} from 'node:crypto'
@@ -30,7 +40,7 @@ const cache = new WeakMap()
  *
  * @this {LoaderContext}
  * @param {string} value
- * @param {(error: Error|null|undefined, content?: string|Buffer, map?: Object) => void} callback
+ * @param {LoaderContext['callback']} callback
  */
 export function loader(value, callback) {
   /** @type {Defaults} */
@@ -69,7 +79,9 @@ export function loader(value, callback) {
 
   process({value, path: this.resourcePath}).then(
     (file) => {
-      callback(null, file.value, file.map || undefined)
+      // @ts-expect-error: `webpack` is not compiled with `exactOptionalPropertyTypes`,
+      // so it does not allow `file.map` to be `undefined` here.
+      callback(null, file.value, file.map)
     },
     (/** @type VFileMessage */ error) => {
       const fpath = path.relative(this.context, this.resourcePath)
diff --git a/packages/loader/package.json b/packages/loader/package.json
index f595e6051..a2ff7e601 100644
--- a/packages/loader/package.json
+++ b/packages/loader/package.json
@@ -39,8 +39,9 @@
   "types": "index.d.ts",
   "files": [
     "lib/",
-    "index.d.ts",
-    "index.cjs"
+    "index.cjs",
+    "index.d.cts",
+    "index.d.ts"
   ],
   "dependencies": {
     "@mdx-js/mdx": "^2.0.0",
@@ -65,7 +66,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/loader/test/index.test.js b/packages/loader/test/index.test.js
index f3165dd45..bf9a2be87 100644
--- a/packages/loader/test/index.test.js
+++ b/packages/loader/test/index.test.js
@@ -2,7 +2,6 @@
  * @typedef {import('mdx/types.js').MDXContent} MDXContent
  * @typedef {import('preact').FunctionComponent<unknown>} PreactComponent
  * @typedef {import('vue').Component} VueComponent
- * @typedef {import('vue').SetupContext} SetupContext
  */
 
 import {promises as fs} from 'fs'
diff --git a/packages/loader/tsconfig.json b/packages/loader/tsconfig.json
index 115ac6895..1a560ac14 100644
--- a/packages/loader/tsconfig.json
+++ b/packages/loader/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx", "index.d.ts", "index.d.cts"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/mdx/lib/compile.js b/packages/mdx/lib/compile.js
index ce1e17034..6d54d2879 100644
--- a/packages/mdx/lib/compile.js
+++ b/packages/mdx/lib/compile.js
@@ -1,14 +1,21 @@
 /**
- * @typedef {import('vfile').VFileCompatible} VFileCompatible
  * @typedef {import('vfile').VFile} VFile
+ * @typedef {import('vfile').VFileCompatible} VFileCompatible
  * @typedef {import('./core.js').PluginOptions} PluginOptions
  * @typedef {import('./core.js').BaseProcessorOptions} BaseProcessorOptions
+ */
+
+/**
  * @typedef {Omit<BaseProcessorOptions, 'format'>} CoreProcessorOptions
+ *   Core configuration.
  *
  * @typedef ExtraOptions
- * @property {'detect'|'mdx'|'md'} [format='detect'] Format of `file`
+ *   Extra configuration.
+ * @property {'detect' | 'mdx' | 'md' | null | undefined} [format='detect']
+ *   Format of `file`.
  *
  * @typedef {CoreProcessorOptions & PluginOptions & ExtraOptions} CompileOptions
+ *   Configuration.
  */
 
 import {createProcessor} from './core.js'
@@ -20,8 +27,10 @@ import {resolveFileAndOptions} from './util/resolve-file-and-options.js'
  * @param {VFileCompatible} vfileCompatible
  *   MDX document to parse (`string`, `Buffer`, `vfile`, anything that can be
  *   given to `vfile`).
- * @param {CompileOptions} [compileOptions]
+ * @param {CompileOptions | null | undefined} [compileOptions]
+ *   Compile configuration.
  * @return {Promise<VFile>}
+ *   File.
  */
 export function compile(vfileCompatible, compileOptions) {
   const {file, options} = resolveFileAndOptions(vfileCompatible, compileOptions)
@@ -34,8 +43,10 @@ export function compile(vfileCompatible, compileOptions) {
  * @param {VFileCompatible} vfileCompatible
  *   MDX document to parse (`string`, `Buffer`, `vfile`, anything that can be
  *   given to `vfile`).
- * @param {CompileOptions} [compileOptions]
+ * @param {CompileOptions | null | undefined} [compileOptions]
+ *   Compile configuration.
  * @return {VFile}
+ *   File.
  */
 export function compileSync(vfileCompatible, compileOptions) {
   const {file, options} = resolveFileAndOptions(vfileCompatible, compileOptions)
diff --git a/packages/mdx/lib/core.js b/packages/mdx/lib/core.js
index 1d3aec731..f028dbfec 100644
--- a/packages/mdx/lib/core.js
+++ b/packages/mdx/lib/core.js
@@ -1,33 +1,39 @@
 /**
- * @typedef {import('unified').Processor} Processor
+ * @typedef {import('remark-rehype').Options} RemarkRehypeOptions
  * @typedef {import('unified').PluggableList} PluggableList
+ * @typedef {import('unified').Processor} Processor
  * @typedef {import('./plugin/recma-document.js').RecmaDocumentOptions} RecmaDocumentOptions
  * @typedef {import('./plugin/recma-stringify.js').RecmaStringifyOptions} RecmaStringifyOptions
  * @typedef {import('./plugin/recma-jsx-rewrite.js').RecmaJsxRewriteOptions} RecmaJsxRewriteOptions
- * @typedef {import('remark-rehype').Options} RemarkRehypeOptions
- *
+ */
+
+/**
  * @typedef BaseProcessorOptions
- * @property {boolean} [jsx=false]
+ *   Base configuration.
+ * @property {boolean | null | undefined} [jsx=false]
  *   Whether to keep JSX.
- * @property {'mdx'|'md'} [format='mdx']
+ * @property {'mdx' | 'md' | null | undefined} [format='mdx']
  *   Format of the files to be processed.
- * @property {'program'|'function-body'} [outputFormat='program']
+ * @property {'function-body' | 'program'} [outputFormat='program']
  *   Whether to compile to a whole program or a function body..
- * @property {Array<string>} [mdExtensions]
+ * @property {Array<string> | null | undefined} [mdExtensions]
  *   Extensions (with `.`) for markdown.
- * @property {Array<string>} [mdxExtensions]
+ * @property {Array<string> | null | undefined} [mdxExtensions]
  *   Extensions (with `.`) for MDX.
- * @property {PluggableList} [recmaPlugins]
+ * @property {PluggableList | null | undefined} [recmaPlugins]
  *   List of recma (esast, JavaScript) plugins.
- * @property {PluggableList} [remarkPlugins]
+ * @property {PluggableList | null | undefined} [remarkPlugins]
  *   List of remark (mdast, markdown) plugins.
- * @property {PluggableList} [rehypePlugins]
+ * @property {PluggableList | null | undefined} [rehypePlugins]
  *   List of rehype (hast, HTML) plugins.
- * @property {RemarkRehypeOptions} [remarkRehypeOptions]
+ * @property {RemarkRehypeOptions | null | undefined} [remarkRehypeOptions]
  *   Options to pass through to `remark-rehype`.
  *
  * @typedef {Omit<RecmaDocumentOptions & RecmaStringifyOptions & RecmaJsxRewriteOptions, 'outputFormat'>} PluginOptions
+ *   Configuration for internal plugins.
+ *
  * @typedef {BaseProcessorOptions & PluginOptions} ProcessorOptions
+ *   Configuration for processor.
  */
 
 import {unified} from 'unified'
@@ -60,12 +66,14 @@ const removedOptions = [
  * 2. Transform through remark (mdast), rehype (hast), and recma (esast)
  * 3. Serialize as JavaScript
  *
- * @param {ProcessorOptions} [options]
+ * @param {ProcessorOptions | null | undefined} [options]
+ *   Configuration.
  * @return {Processor}
+ *   Processor.
  */
-export function createProcessor(options = {}) {
+export function createProcessor(options) {
   const {
-    development = defaultDevelopment,
+    development,
     jsx,
     format,
     outputFormat,
@@ -73,15 +81,19 @@ export function createProcessor(options = {}) {
     recmaPlugins,
     rehypePlugins,
     remarkPlugins,
-    remarkRehypeOptions = {},
+    remarkRehypeOptions,
     SourceMapGenerator,
     ...rest
-  } = options
+  } = options || {}
+  const dev =
+    development === null || development === undefined
+      ? defaultDevelopment
+      : development
   let index = -1
 
   while (++index < removedOptions.length) {
     const key = removedOptions[index]
-    if (key in options) {
+    if (options && key in options) {
       throw new Error(
         '`options.' +
           key +
@@ -104,14 +116,18 @@ export function createProcessor(options = {}) {
     pipeline.use(remarkMdx)
   }
 
+  const extraNodeTypes = remarkRehypeOptions
+    ? /* c8 ignore next */
+      remarkRehypeOptions.passThrough || []
+    : []
+
   pipeline
     .use(remarkMarkAndUnravel)
     .use(remarkPlugins || [])
     .use(remarkRehype, {
       ...remarkRehypeOptions,
       allowDangerousHtml: true,
-      /* c8 ignore next */
-      passThrough: [...(remarkRehypeOptions.passThrough || []), ...nodeTypes]
+      passThrough: [...extraNodeTypes, ...nodeTypes]
     })
     .use(rehypePlugins || [])
 
@@ -122,10 +138,14 @@ export function createProcessor(options = {}) {
   pipeline
     .use(rehypeRecma)
     .use(recmaDocument, {...rest, outputFormat})
-    .use(recmaJsxRewrite, {development, providerImportSource, outputFormat})
+    .use(recmaJsxRewrite, {
+      development: dev,
+      providerImportSource,
+      outputFormat
+    })
 
   if (!jsx) {
-    pipeline.use(recmaJsxBuild, {development, outputFormat})
+    pipeline.use(recmaJsxBuild, {development: dev, outputFormat})
   }
 
   pipeline.use(recmaStringify, {SourceMapGenerator}).use(recmaPlugins || [])
diff --git a/packages/mdx/lib/evaluate.js b/packages/mdx/lib/evaluate.js
index 3a17b98d7..72e0bc7a9 100644
--- a/packages/mdx/lib/evaluate.js
+++ b/packages/mdx/lib/evaluate.js
@@ -1,8 +1,7 @@
 /**
+ * @typedef {import('mdx/types.js').MDXModule} ExportMap
  * @typedef {import('vfile').VFileCompatible} VFileCompatible
  * @typedef {import('./util/resolve-evaluate-options.js').EvaluateOptions} EvaluateOptions
- *
- * @typedef {import('mdx/types.js').MDXModule} ExportMap
  */
 
 import {compile, compileSync} from './compile.js'
@@ -16,7 +15,9 @@ import {resolveEvaluateOptions} from './util/resolve-evaluate-options.js'
  *   MDX document to parse (`string`, `Buffer`, `vfile`, anything that can be
  *   given to `vfile`).
  * @param {EvaluateOptions} evaluateOptions
+ *   Configuration for evaluation.
  * @return {Promise<ExportMap>}
+ *   Export map.
  */
 export async function evaluate(vfileCompatible, evaluateOptions) {
   const {compiletime, runtime} = resolveEvaluateOptions(evaluateOptions)
@@ -32,7 +33,9 @@ export async function evaluate(vfileCompatible, evaluateOptions) {
  *   MDX document to parse (`string`, `Buffer`, `vfile`, anything that can be
  *   given to `vfile`).
  * @param {EvaluateOptions} evaluateOptions
+ *   Configuration for evaluation.
  * @return {ExportMap}
+ *   Export map.
  */
 export function evaluateSync(vfileCompatible, evaluateOptions) {
   const {compiletime, runtime} = resolveEvaluateOptions(evaluateOptions)
diff --git a/packages/mdx/lib/plugin/recma-document.js b/packages/mdx/lib/plugin/recma-document.js
index 82996bb90..156b57153 100644
--- a/packages/mdx/lib/plugin/recma-document.js
+++ b/packages/mdx/lib/plugin/recma-document.js
@@ -1,9 +1,9 @@
 /**
  * @typedef {import('estree-jsx').Directive} Directive
+ * @typedef {import('estree-jsx').ExportAllDeclaration} ExportAllDeclaration
  * @typedef {import('estree-jsx').ExportDefaultDeclaration} ExportDefaultDeclaration
- * @typedef {import('estree-jsx').ExportSpecifier} ExportSpecifier
  * @typedef {import('estree-jsx').ExportNamedDeclaration} ExportNamedDeclaration
- * @typedef {import('estree-jsx').ExportAllDeclaration} ExportAllDeclaration
+ * @typedef {import('estree-jsx').ExportSpecifier} ExportSpecifier
  * @typedef {import('estree-jsx').Expression} Expression
  * @typedef {import('estree-jsx').FunctionDeclaration} FunctionDeclaration
  * @typedef {import('estree-jsx').ImportDeclaration} ImportDeclaration
@@ -11,32 +11,35 @@
  * @typedef {import('estree-jsx').ModuleDeclaration} ModuleDeclaration
  * @typedef {import('estree-jsx').Node} Node
  * @typedef {import('estree-jsx').Program} Program
+ * @typedef {import('estree-jsx').Property} Property
  * @typedef {import('estree-jsx').SimpleLiteral} SimpleLiteral
+ * @typedef {import('estree-jsx').SpreadElement} SpreadElement
  * @typedef {import('estree-jsx').Statement} Statement
  * @typedef {import('estree-jsx').VariableDeclarator} VariableDeclarator
- * @typedef {import('estree-jsx').SpreadElement} SpreadElement
- * @typedef {import('estree-jsx').Property} Property
- *
+ */
+
+/**
  * @typedef RecmaDocumentOptions
- * @property {'program'|'function-body'} [outputFormat='program']
+ *   Configuration for internal plugin `recma-document`.
+ * @property {'function-body' | 'program' | null | undefined} [outputFormat='program']
  *   Whether to use either `import` and `export` statements to get the runtime
  *   (and optionally provider) and export the content, or get values from
  *   `arguments` and return things.
- * @property {boolean} [useDynamicImport=false]
+ * @property {boolean | null | undefined} [useDynamicImport=false]
  *   Whether to keep `import` (and `export … from`) statements or compile them
  *   to dynamic `import()` instead.
- * @property {string} [baseUrl]
+ * @property {string | null | undefined} [baseUrl]
  *   Resolve `import`s (and `export … from`, and `import.meta.url`) relative to
  *   this URL.
- * @property {string} [pragma='React.createElement']
+ * @property {string | null | undefined} [pragma='React.createElement']
  *   Pragma for JSX (used in classic runtime).
- * @property {string} [pragmaFrag='React.Fragment']
+ * @property {string | null | undefined} [pragmaFrag='React.Fragment']
  *   Pragma for JSX fragments (used in classic runtime).
- * @property {string} [pragmaImportSource='react']
+ * @property {string | null | undefined} [pragmaImportSource='react']
  *   Where to import the identifier of `pragma` from (used in classic runtime).
- * @property {string} [jsxImportSource='react']
+ * @property {string | null | undefined} [jsxImportSource='react']
  *   Place to import automatic JSX runtimes from (used in automatic runtime).
- * @property {'automatic'|'classic'} [jsxRuntime='automatic']
+ * @property {'automatic' | 'classic' | null | undefined} [jsxRuntime='automatic']
  *   JSX runtime to use.
  */
 
@@ -52,31 +55,34 @@ import {isDeclaration} from '../util/estree-util-is-declaration.js'
 /**
  * A plugin to wrap the estree in `MDXContent`.
  *
- * @type {import('unified').Plugin<[RecmaDocumentOptions]|[], Program>}
+ * @type {import('unified').Plugin<[RecmaDocumentOptions | null | undefined] | [], Program>}
  */
-export function recmaDocument(options = {}) {
-  const {
-    baseUrl,
-    useDynamicImport,
-    outputFormat = 'program',
-    pragma = 'React.createElement',
-    pragmaFrag = 'React.Fragment',
-    pragmaImportSource = 'react',
-    jsxImportSource = 'react',
-    jsxRuntime = 'automatic'
-  } = options
+export function recmaDocument(options) {
+  // Always given inside `@mdx-js/mdx`
+  /* c8 ignore next */
+  const options_ = options || {}
+  const baseUrl = options_.baseUrl || undefined
+  const useDynamicImport = options_.useDynamicImport || undefined
+  const outputFormat = options_.outputFormat || 'program'
+  const pragma =
+    options_.pragma === undefined ? 'React.createElement' : options_.pragma
+  const pragmaFrag =
+    options_.pragmaFrag === undefined ? 'React.Fragment' : options_.pragmaFrag
+  const pragmaImportSource = options_.pragmaImportSource || 'react'
+  const jsxImportSource = options_.jsxImportSource || 'react'
+  const jsxRuntime = options_.jsxRuntime || 'automatic'
 
   return (tree, file) => {
-    /** @type {Array<string|[string, string]>} */
+    /** @type {Array<[string, string] | string>} */
     const exportedIdentifiers = []
-    /** @type {Array<Directive|Statement|ModuleDeclaration>} */
+    /** @type {Array<Directive | ModuleDeclaration | Statement>} */
     const replacement = []
     /** @type {Array<string>} */
     const pragmas = []
     let exportAllCount = 0
-    /** @type {ExportDefaultDeclaration|ExportSpecifier|undefined} */
+    /** @type {ExportDefaultDeclaration | ExportSpecifier | undefined} */
     let layout
-    /** @type {boolean|undefined} */
+    /** @type {boolean | undefined} */
     let content
     /** @type {Node} */
     let child
@@ -298,9 +304,7 @@ export function recmaDocument(options = {}) {
 
     if (baseUrl) {
       walk(tree, {
-        enter(_node) {
-          const node = /** @type {Node} */ (_node)
-
+        enter(node) {
           if (
             node.type === 'MemberExpression' &&
             'object' in node &&
@@ -319,7 +323,7 @@ export function recmaDocument(options = {}) {
     }
 
     /**
-     * @param {ExportNamedDeclaration|ExportAllDeclaration} node
+     * @param {ExportAllDeclaration | ExportNamedDeclaration} node
      * @returns {void}
      */
     function handleExport(node) {
@@ -348,7 +352,7 @@ export function recmaDocument(options = {}) {
     }
 
     /**
-     * @param {ImportDeclaration|ExportNamedDeclaration|ExportAllDeclaration} node
+     * @param {ExportAllDeclaration | ExportNamedDeclaration | ImportDeclaration} node
      * @returns {void}
      */
     function handleEsm(node) {
@@ -375,7 +379,7 @@ export function recmaDocument(options = {}) {
         node.source = create(node.source, {type: 'Literal', value})
       }
 
-      /** @type {Statement|ModuleDeclaration|undefined} */
+      /** @type {ModuleDeclaration | Statement | undefined} */
       let replace
       /** @type {Expression} */
       let init
@@ -478,9 +482,9 @@ export function recmaDocument(options = {}) {
   }
 
   /**
-   * @param {Expression} [content]
-   * @param {boolean} [hasInternalLayout]
-   * @returns {FunctionDeclaration[]}
+   * @param {Expression | undefined} [content]
+   * @param {boolean | undefined} [hasInternalLayout]
+   * @returns {Array<FunctionDeclaration>}
    */
   function createMdxContent(content, hasInternalLayout) {
     /** @type {JSXElement} */
@@ -575,12 +579,7 @@ export function recmaDocument(options = {}) {
         ],
         body: {
           type: 'BlockStatement',
-          body: [
-            {
-              type: 'ReturnStatement',
-              argument: result
-            }
-          ]
+          body: [{type: 'ReturnStatement', argument: result}]
         }
       }
     ]
diff --git a/packages/mdx/lib/plugin/recma-jsx-build.js b/packages/mdx/lib/plugin/recma-jsx-build.js
index f7e14b021..630c6230e 100644
--- a/packages/mdx/lib/plugin/recma-jsx-build.js
+++ b/packages/mdx/lib/plugin/recma-jsx-build.js
@@ -1,11 +1,16 @@
 /**
  * @typedef {import('estree-jsx').Program} Program
  * @typedef {import('estree-util-build-jsx').BuildJsxOptions} BuildJsxOptions
- *
- * @typedef RecmaJsxBuildOptions
- * @property {'program'|'function-body'} [outputFormat='program']
+ */
+
+/**
+ * @typedef ExtraOptions
+ *   Configuration for internal plugin `recma-jsx-build`.
+ * @property {'function-body' | 'program' | null | undefined} [outputFormat='program']
  *   Whether to keep the import of the automatic runtime or get it from
  *   `arguments[0]` instead.
+ *
+ * @typedef {BuildJsxOptions & ExtraOptions} RecmaJsxBuildOptions
  */
 
 import {buildJsx} from 'estree-util-build-jsx'
@@ -16,10 +21,12 @@ import {toIdOrMemberExpression} from '../util/estree-util-to-id-or-member-expres
  * A plugin to build JSX into function calls.
  * `estree-util-build-jsx` does all the work for us!
  *
- * @type {import('unified').Plugin<[BuildJsxOptions & RecmaJsxBuildOptions?], Program>}
+ * @type {import('unified').Plugin<[RecmaJsxBuildOptions | null | undefined] | [], Program>}
  */
-export function recmaJsxBuild(options = {}) {
-  const {development, outputFormat} = options
+export function recmaJsxBuild(options) {
+  // Always given inside `@mdx-js/mdx`
+  /* c8 ignore next */
+  const {development, outputFormat} = options || {}
 
   return (tree, file) => {
     buildJsx(tree, {development, filePath: file.history[0]})
diff --git a/packages/mdx/lib/plugin/recma-jsx-rewrite.js b/packages/mdx/lib/plugin/recma-jsx-rewrite.js
index 35aeca805..50dd37a19 100644
--- a/packages/mdx/lib/plugin/recma-jsx-rewrite.js
+++ b/packages/mdx/lib/plugin/recma-jsx-rewrite.js
@@ -1,30 +1,30 @@
 /**
- * @typedef {import('estree-jsx').Node} Node
  * @typedef {import('estree-jsx').Expression} Expression
- * @typedef {import('estree-jsx').Function} ESFunction
+ * @typedef {import('estree-jsx').Function} EstreeFunction
+ * @typedef {import('estree-jsx').Identifier} Identifier
  * @typedef {import('estree-jsx').ImportSpecifier} ImportSpecifier
  * @typedef {import('estree-jsx').JSXElement} JSXElement
- * @typedef {import('estree-jsx').JSXIdentifier} JSXIdentifier
- * @typedef {import('estree-jsx').JSXNamespacedName} JSXNamespacedName
  * @typedef {import('estree-jsx').ModuleDeclaration} ModuleDeclaration
+ * @typedef {import('estree-jsx').Node} Node
+ * @typedef {import('estree-jsx').ObjectPattern} ObjectPattern
  * @typedef {import('estree-jsx').Program} Program
  * @typedef {import('estree-jsx').Property} Property
  * @typedef {import('estree-jsx').Statement} Statement
  * @typedef {import('estree-jsx').VariableDeclarator} VariableDeclarator
- * @typedef {import('estree-jsx').ObjectPattern} ObjectPattern
- * @typedef {import('estree-jsx').Identifier} Identifier
- *
- * @typedef {import('estree-walker').SyncHandler} WalkHandler
  *
  * @typedef {import('periscopic').Scope & {node: Node}} Scope
- *
+ */
+
+/**
  * @typedef RecmaJsxRewriteOptions
- * @property {'program'|'function-body'} [outputFormat='program']
+ *   Configuration for internal plugin `recma-jsx-rewrite`.
+ * @property {'function-body' | 'program' | null | undefined} [outputFormat='program']
  *   Whether to use an import statement or `arguments[0]` to get the provider.
- * @property {string} [providerImportSource]
+ * @property {string | null | undefined} [providerImportSource]
  *   Place to import a provider from.
- * @property {boolean} [development=false]
+ * @property {boolean | null | undefined} [development=false]
  *   Whether to add extra info to error messages in generated code.
+ *
  *   This also results in the development automatic JSX runtime
  *   (`/jsx-dev-runtime`, `jsxDEV`) being used, which passes positional info to
  *   nodes.
@@ -37,7 +37,7 @@
  * @property {Array<string>} tags
  * @property {Record<string, {node: JSXElement, component: boolean}>} references
  * @property {Map<string|number, string>} idToInvalidComponentName
- * @property {ESFunction} node
+ * @property {EstreeFunction} node
  */
 
 import {stringifyPosition} from 'unist-util-stringify-position'
@@ -61,27 +61,26 @@ const own = {}.hasOwnProperty
  * It also makes sure that any undefined components are defined: either from
  * received components or as a function that throws an error.
  *
- * @type {import('unified').Plugin<[RecmaJsxRewriteOptions]|[], Program>}
+ * @type {import('unified').Plugin<[RecmaJsxRewriteOptions | null | undefined] | [], Program>}
  */
-export function recmaJsxRewrite(options = {}) {
-  const {development, providerImportSource, outputFormat} = options
+export function recmaJsxRewrite(options) {
+  // Always given inside `@mdx-js/mdx`
+  /* c8 ignore next */
+  const {development, providerImportSource, outputFormat} = options || {}
 
   return (tree, file) => {
     // Find everything that’s defined in the top-level scope.
     const scopeInfo = analyze(tree)
     /** @type {Array<StackEntry>} */
     const fnStack = []
-    /** @type {boolean|undefined} */
-    let importProvider
-    /** @type {boolean|undefined} */
-    let createErrorHelper
-    /** @type {Scope|null} */
+    let importProvider = false
+    let createErrorHelper = false
+    /** @type {Scope | undefined} */
     let currentScope
 
     walk(tree, {
-      enter(_node) {
-        const node = /** @type {Node} */ (_node)
-        const newScope = /** @type {Scope|undefined} */ (
+      enter(node) {
+        const newScope = /** @type {Scope | undefined} */ (
           scopeInfo.map.get(node)
         )
 
@@ -144,7 +143,7 @@ export function recmaJsxRewrite(options = {}) {
             const isInScope = inScope(currentScope, id)
 
             if (!own.call(fnScope.references, fullId)) {
-              const parentScope = /** @type {Scope|null} */ (
+              const parentScope = /** @type {Scope | undefined} */ (
                 currentScope.parent
               )
               if (
@@ -242,7 +241,7 @@ export function recmaJsxRewrite(options = {}) {
           node.type === 'FunctionExpression' ||
           node.type === 'ArrowFunctionExpression'
         ) {
-          const fn = /** @type {ESFunction} */ (node)
+          const fn = node
           const scope = fnStack[fnStack.length - 1]
           /** @type {string} */
           let name
@@ -326,7 +325,7 @@ export function recmaJsxRewrite(options = {}) {
                   }
                 : parameters[0]
 
-            /** @type {ObjectPattern|undefined} */
+            /** @type {ObjectPattern | undefined} */
             let componentsPattern
 
             // Add components to scope.
@@ -558,7 +557,7 @@ export function recmaJsxRewrite(options = {}) {
 /**
  * @param {string} providerImportSource
  * @param {RecmaJsxRewriteOptions['outputFormat']} outputFormat
- * @returns {Statement|ModuleDeclaration}
+ * @returns {Statement | ModuleDeclaration}
  */
 function createImportProvider(providerImportSource, outputFormat) {
   /** @type {Array<ImportSpecifier>} */
@@ -587,7 +586,7 @@ function createImportProvider(providerImportSource, outputFormat) {
 }
 
 /**
- * @param {ESFunction} node
+ * @param {EstreeFunction} node
  * @param {string} name
  * @returns {boolean}
  */
@@ -598,9 +597,10 @@ function isNamedFunction(node, name) {
 /**
  * @param {Scope} scope
  * @param {string} id
+ * @returns {boolean}
  */
 function inScope(scope, id) {
-  /** @type {Scope|null} */
+  /** @type {Scope | undefined} */
   let currentScope = scope
 
   while (currentScope) {
diff --git a/packages/mdx/lib/plugin/recma-stringify.js b/packages/mdx/lib/plugin/recma-stringify.js
index 9f8e6477f..b1c99bfa9 100644
--- a/packages/mdx/lib/plugin/recma-stringify.js
+++ b/packages/mdx/lib/plugin/recma-stringify.js
@@ -3,7 +3,8 @@
  * @typedef {typeof import('source-map').SourceMapGenerator} SourceMapGenerator
  *
  * @typedef RecmaStringifyOptions
- * @property {SourceMapGenerator} [SourceMapGenerator]
+ *   Configuration for internal plugin `recma-stringify`.
+ * @property {SourceMapGenerator | null | undefined} [SourceMapGenerator]
  *   Generate a source map by passing a `SourceMapGenerator` from `source-map`
  *   in.
  */
@@ -15,10 +16,12 @@ import {toJs, jsx} from 'estree-util-to-js'
  * support for serializing JSX.
  *
  * @this {import('unified').Processor}
- * @type {import('unified').Plugin<[RecmaStringifyOptions]|[], Program, string>}
+ * @type {import('unified').Plugin<[RecmaStringifyOptions | null | undefined] | [], Program, string>}
  */
-export function recmaStringify(options = {}) {
-  const {SourceMapGenerator} = options
+export function recmaStringify(options) {
+  // Always given inside `@mdx-js/mdx`
+  /* c8 ignore next */
+  const {SourceMapGenerator} = options || {}
 
   Object.assign(this, {Compiler: compiler})
 
diff --git a/packages/mdx/lib/plugin/rehype-recma.js b/packages/mdx/lib/plugin/rehype-recma.js
index 4acec9a2e..a55abbc31 100644
--- a/packages/mdx/lib/plugin/rehype-recma.js
+++ b/packages/mdx/lib/plugin/rehype-recma.js
@@ -9,7 +9,7 @@ import {toEstree} from 'hast-util-to-estree'
  * A plugin to transform an HTML (hast) tree to a JS (estree).
  * `hast-util-to-estree` does all the work for us!
  *
- * @type {import('unified').Plugin<Array<void>, Root, Program>}
+ * @type {import('unified').Plugin<[], Root, Program>}
  */
 export function rehypeRecma() {
   return (tree) => toEstree(tree)
diff --git a/packages/mdx/lib/plugin/rehype-remove-raw.js b/packages/mdx/lib/plugin/rehype-remove-raw.js
index bf660b69f..098f4c0d2 100644
--- a/packages/mdx/lib/plugin/rehype-remove-raw.js
+++ b/packages/mdx/lib/plugin/rehype-remove-raw.js
@@ -7,10 +7,11 @@ import {visit} from 'unist-util-visit'
 
 /**
  * A tiny plugin that removes raw HTML.
+ *
  * This is needed if the format is `md` and `rehype-raw` was not used to parse
  * dangerous HTML into nodes.
  *
- * @type {import('unified').Plugin<Array<void>, Root>}
+ * @type {import('unified').Plugin<[], Root>}
  */
 export function rehypeRemoveRaw() {
   return (tree) => {
diff --git a/packages/mdx/lib/plugin/remark-mark-and-unravel.js b/packages/mdx/lib/plugin/remark-mark-and-unravel.js
index 53c3f929e..68e6eb76a 100644
--- a/packages/mdx/lib/plugin/remark-mark-and-unravel.js
+++ b/packages/mdx/lib/plugin/remark-mark-and-unravel.js
@@ -1,8 +1,5 @@
 /**
  * @typedef {import('mdast').Root} Root
- * @typedef {import('mdast').Content} Content
- * @typedef {Root|Content} Node
- * @typedef {Extract<Node, import('unist').Parent>} Parent
  *
  * @typedef {import('remark-mdx')} DoNotTouchAsThisImportItIncludesMdxInTree
  */
@@ -12,19 +9,18 @@ import {visit} from 'unist-util-visit'
 /**
  * A tiny plugin that unravels `<p><h1>x</h1></p>` but also
  * `<p><Component /></p>` (so it has no knowledge of “HTML”).
+ *
  * It also marks JSX as being explicitly JSX, so when a user passes a `h1`
  * component, it is used for `# heading` but not for `<h1>heading</h1>`.
  *
- * @type {import('unified').Plugin<Array<void>, Root>}
+ * @type {import('unified').Plugin<[], Root>}
  */
 export function remarkMarkAndUnravel() {
   return (tree) => {
-    visit(tree, (node, index, parent_) => {
-      const parent = /** @type {Parent} */ (parent_)
+    visit(tree, (node, index, parent) => {
       let offset = -1
       let all = true
-      /** @type {boolean|undefined} */
-      let oneOrMore
+      let oneOrMore = false
 
       if (parent && typeof index === 'number' && node.type === 'paragraph') {
         const children = node.children
diff --git a/packages/mdx/lib/run.js b/packages/mdx/lib/run.js
index 80ecfec9b..aab96cbb2 100644
--- a/packages/mdx/lib/run.js
+++ b/packages/mdx/lib/run.js
@@ -7,7 +7,9 @@ const AsyncFunction = Object.getPrototypeOf(run).constructor
  * @param {{toString(): string}} file
  *   JS document to run.
  * @param {unknown} options
- * @return {Promise<*>}
+ *   Parameter.
+ * @return {Promise<any>}
+ *   Anthing.
  */
 export async function run(file, options) {
   // V8 on Erbium.
@@ -21,7 +23,9 @@ export async function run(file, options) {
  * @param {{toString(): string}} file
  *   JS document to run.
  * @param {unknown} options
- * @return {*}
+ *   Parameter.
+ * @return {any}
+ *   Anthing.
  */
 export function runSync(file, options) {
   // eslint-disable-next-line no-new-func
diff --git a/packages/mdx/lib/util/create-format-aware-processors.js b/packages/mdx/lib/util/create-format-aware-processors.js
index c17d01d78..ed7abbeb3 100644
--- a/packages/mdx/lib/util/create-format-aware-processors.js
+++ b/packages/mdx/lib/util/create-format-aware-processors.js
@@ -1,7 +1,7 @@
 /**
- * @typedef {import('vfile').VFileCompatible} VFileCompatible
- * @typedef {import('vfile').VFile} VFile
  * @typedef {import('unified').Processor} Processor
+ * @typedef {import('vfile').VFile} VFile
+ * @typedef {import('vfile').VFileCompatible} VFileCompatible
  * @typedef {import('../compile.js').CompileOptions} CompileOptions
  */
 
@@ -12,12 +12,15 @@ import {resolveFileAndOptions} from './resolve-file-and-options.js'
 /**
  * Create smart processors to handle different formats.
  *
- * @param {CompileOptions} [compileOptions]
+ * @param {CompileOptions | null | undefined} [compileOptions]
+ *   configuration.
  * @return {{extnames: Array<string>, process: process, processSync: processSync}}
+ *   Smart processor.
  */
-export function createFormatAwareProcessors(compileOptions = {}) {
-  const mdExtensions = compileOptions.mdExtensions || md
-  const mdxExtensions = compileOptions.mdxExtensions || mdx
+export function createFormatAwareProcessors(compileOptions) {
+  const compileOptions_ = compileOptions || {}
+  const mdExtensions = compileOptions_.mdExtensions || md
+  const mdxExtensions = compileOptions_.mdxExtensions || mdx
   /** @type {Processor} */
   let cachedMarkdown
   /** @type {Processor} */
@@ -25,9 +28,9 @@ export function createFormatAwareProcessors(compileOptions = {}) {
 
   return {
     extnames:
-      compileOptions.format === 'md'
+      compileOptions_.format === 'md'
         ? mdExtensions
-        : compileOptions.format === 'mdx'
+        : compileOptions_.format === 'mdx'
         ? mdxExtensions
         : mdExtensions.concat(mdxExtensions),
     process,
@@ -40,6 +43,7 @@ export function createFormatAwareProcessors(compileOptions = {}) {
    * @param {VFileCompatible} vfileCompatible
    *   MDX or markdown document.
    * @return {Promise<VFile>}
+   *   File.
    */
   function process(vfileCompatible) {
     const {file, processor} = split(vfileCompatible)
@@ -52,6 +56,7 @@ export function createFormatAwareProcessors(compileOptions = {}) {
    * @param {VFileCompatible} vfileCompatible
    *   MDX or markdown document.
    * @return {VFile}
+   *   File.
    */
   // C8 does not cover `.cjs` files (this is only used for the require hook,
   // which has to be CJS).
@@ -70,11 +75,12 @@ export function createFormatAwareProcessors(compileOptions = {}) {
    * @param {VFileCompatible} vfileCompatible
    *   MDX or markdown document.
    * @return {{file: VFile, processor: Processor}}
+   *   File and corresponding processor.
    */
   function split(vfileCompatible) {
     const {file, options} = resolveFileAndOptions(
       vfileCompatible,
-      compileOptions
+      compileOptions_
     )
     const processor =
       options.format === 'md'
diff --git a/packages/mdx/lib/util/estree-util-declaration-to-expression.js b/packages/mdx/lib/util/estree-util-declaration-to-expression.js
index 2f3496194..fa0c2e312 100644
--- a/packages/mdx/lib/util/estree-util-declaration-to-expression.js
+++ b/packages/mdx/lib/util/estree-util-declaration-to-expression.js
@@ -5,12 +5,15 @@
 
 /**
  * Turn a declaration into an expression.
+ *
  * Doesn’t work for variable declarations, but that’s fine for our use case
  * because currently we’re using this utility for export default declarations,
  * which can’t contain variable declarations.
  *
  * @param {Declaration} declaration
+ *   Declaration.
  * @returns {Expression}
+ *   Expression.
  */
 export function declarationToExpression(declaration) {
   if (declaration.type === 'FunctionDeclaration') {
diff --git a/packages/mdx/lib/util/estree-util-is-declaration.js b/packages/mdx/lib/util/estree-util-is-declaration.js
index d2f4254c3..cf2188456 100644
--- a/packages/mdx/lib/util/estree-util-is-declaration.js
+++ b/packages/mdx/lib/util/estree-util-is-declaration.js
@@ -1,18 +1,20 @@
 /**
+ * @typedef {import('estree-jsx').Node} Node
  * @typedef {import('estree-jsx').Declaration} Declaration
  */
 
 /**
- * @param {unknown} node
+ * Check if `node` is a declaration.
+ *
+ * @param {Node} node
+ *   Node to check.
  * @returns {node is Declaration}
+ *   Whether `node` is a declaration.
  */
 export function isDeclaration(node) {
-  /** @type {string} */
-  // @ts-expect-error Hush typescript, looks like `type` is available.
-  const type = node && typeof node === 'object' && node.type
   return Boolean(
-    type === 'FunctionDeclaration' ||
-      type === 'ClassDeclaration' ||
-      type === 'VariableDeclaration'
+    node.type === 'FunctionDeclaration' ||
+      node.type === 'ClassDeclaration' ||
+      node.type === 'VariableDeclaration'
   )
 }
diff --git a/packages/mdx/lib/util/estree-util-specifiers-to-declarations.js b/packages/mdx/lib/util/estree-util-specifiers-to-declarations.js
index 2f75a2c03..25fac38f1 100644
--- a/packages/mdx/lib/util/estree-util-specifiers-to-declarations.js
+++ b/packages/mdx/lib/util/estree-util-specifiers-to-declarations.js
@@ -1,18 +1,17 @@
 /**
+ * @typedef {import('estree-jsx').ExportSpecifier} ExportSpecifier
+ * @typedef {import('estree-jsx').Expression} Expression
  * @typedef {import('estree-jsx').Identifier} Identifier
  * @typedef {import('estree-jsx').ImportSpecifier} ImportSpecifier
  * @typedef {import('estree-jsx').ImportDefaultSpecifier} ImportDefaultSpecifier
  * @typedef {import('estree-jsx').ImportNamespaceSpecifier} ImportNamespaceSpecifier
- * @typedef {import('estree-jsx').ExportSpecifier} ExportSpecifier
- * @typedef {import('estree-jsx').ObjectPattern} ObjectPattern
  * @typedef {import('estree-jsx').VariableDeclarator} VariableDeclarator
- * @typedef {import('estree-jsx').Expression} Expression
  */
 
 import {create} from './estree-util-create.js'
 
 /**
- * @param {Array<ImportSpecifier|ImportDefaultSpecifier|ImportNamespaceSpecifier|ExportSpecifier>} specifiers
+ * @param {Array<ImportSpecifier | ImportDefaultSpecifier | ImportNamespaceSpecifier | ExportSpecifier>} specifiers
  * @param {Expression} init
  * @returns {Array<VariableDeclarator>}
  */
@@ -20,10 +19,10 @@ export function specifiersToDeclarations(specifiers, init) {
   let index = -1
   /** @type {Array<VariableDeclarator>} */
   const declarations = []
-  /** @type {Array<ImportSpecifier|ImportDefaultSpecifier|ExportSpecifier>} */
+  /** @type {Array<ImportSpecifier | ImportDefaultSpecifier | ExportSpecifier>} */
   const otherSpecifiers = []
   // Can only be one according to JS syntax.
-  /** @type {ImportNamespaceSpecifier|undefined} */
+  /** @type {ImportNamespaceSpecifier | undefined} */
   let importNamespaceSpecifier
 
   while (++index < specifiers.length) {
diff --git a/packages/mdx/lib/util/estree-util-to-binary-addition.js b/packages/mdx/lib/util/estree-util-to-binary-addition.js
index 6d36b28b5..1f5744f99 100644
--- a/packages/mdx/lib/util/estree-util-to-binary-addition.js
+++ b/packages/mdx/lib/util/estree-util-to-binary-addition.js
@@ -7,7 +7,7 @@
  */
 export function toBinaryAddition(expressions) {
   let index = -1
-  /** @type {Expression|undefined} */
+  /** @type {Expression | undefined} */
   let left
 
   while (++index < expressions.length) {
diff --git a/packages/mdx/lib/util/estree-util-to-id-or-member-expression.js b/packages/mdx/lib/util/estree-util-to-id-or-member-expression.js
index 63fe3f1fd..41b0d4aad 100644
--- a/packages/mdx/lib/util/estree-util-to-id-or-member-expression.js
+++ b/packages/mdx/lib/util/estree-util-to-id-or-member-expression.js
@@ -1,9 +1,9 @@
 /**
  * @typedef {import('estree-jsx').Identifier} Identifier
- * @typedef {import('estree-jsx').Literal} Literal
  * @typedef {import('estree-jsx').JSXIdentifier} JSXIdentifier
- * @typedef {import('estree-jsx').MemberExpression} MemberExpression
  * @typedef {import('estree-jsx').JSXMemberExpression} JSXMemberExpression
+ * @typedef {import('estree-jsx').Literal} Literal
+ * @typedef {import('estree-jsx').MemberExpression} MemberExpression
  */
 
 import {
@@ -20,7 +20,7 @@ export const toIdOrMemberExpression = toIdOrMemberExpressionFactory(
 
 export const toJsxIdOrMemberExpression =
   // @ts-expect-error: fine
-  /** @type {(ids: Array<string|number>) => JSXIdentifier|JSXMemberExpression)} */
+  /** @type {(ids: Array<string | number>) => JSXIdentifier | JSXMemberExpression)} */
   (
     toIdOrMemberExpressionFactory(
       'JSXIdentifier',
@@ -37,12 +37,12 @@ export const toJsxIdOrMemberExpression =
 function toIdOrMemberExpressionFactory(idType, memberType, isIdentifier) {
   return toIdOrMemberExpression
   /**
-   * @param {Array<string|number>} ids
-   * @returns {Identifier|MemberExpression}
+   * @param {Array<string | number>} ids
+   * @returns {Identifier | MemberExpression}
    */
   function toIdOrMemberExpression(ids) {
     let index = -1
-    /** @type {Identifier|Literal|MemberExpression|undefined} */
+    /** @type {Identifier | Literal | MemberExpression | undefined} */
     let object
 
     while (++index < ids.length) {
@@ -56,7 +56,7 @@ function toIdOrMemberExpressionFactory(idType, memberType, isIdentifier) {
         throw new Error('Cannot turn `' + name + '` into a JSX identifier')
       }
 
-      /** @type {Identifier|Literal} */
+      /** @type {Identifier | Literal} */
       // @ts-expect-error: JSX is fine.
       const id = valid ? {type: idType, name} : {type: 'Literal', value: name}
       // @ts-expect-error: JSX is fine.
diff --git a/packages/mdx/lib/util/resolve-evaluate-options.js b/packages/mdx/lib/util/resolve-evaluate-options.js
index de73f412e..355cf7efd 100644
--- a/packages/mdx/lib/util/resolve-evaluate-options.js
+++ b/packages/mdx/lib/util/resolve-evaluate-options.js
@@ -2,26 +2,29 @@
  * @typedef {import('../core.js').ProcessorOptions} ProcessorOptions
  *
  * @typedef RunnerOptions
- * @property {*} Fragment
+ *   Configuration with JSX runtime.
+ * @property {any} Fragment
  *   Symbol to use for fragments.
- * @property {*} [jsx]
+ * @property {any} [jsx]
  *   Function to generate an element with static children in production mode.
- * @property {*} [jsxs]
+ * @property {any} [jsxs]
  *   Function to generate an element with dynamic children in production mode.
- * @property {*} [jsxDEV]
+ * @property {any} [jsxDEV]
  *   Function to generate an element in development mode.
- * @property {*} [useMDXComponents]
+ * @property {any} [useMDXComponents]
  *   Function to get `MDXComponents` from context.
  *
  * @typedef {Omit<ProcessorOptions, 'jsx' | 'jsxImportSource' | 'jsxRuntime' | 'pragma' | 'pragmaFrag' | 'pragmaImportSource' | 'providerImportSource' | 'outputFormat'> } EvaluateProcessorOptions
+ *   Compile configuration without JSX options for evaluation.
  *
  * @typedef {EvaluateProcessorOptions & RunnerOptions} EvaluateOptions
+ *   Configuration for evaluation.
  */
 
 /**
  * Split compiletime options from runtime options.
  *
- * @param {EvaluateOptions} options
+ * @param {EvaluateOptions | null | undefined} options
  * @returns {{compiletime: ProcessorOptions, runtime: RunnerOptions}}
  */
 export function resolveEvaluateOptions(options) {
diff --git a/packages/mdx/lib/util/resolve-file-and-options.js b/packages/mdx/lib/util/resolve-file-and-options.js
index c5d08331d..143d54c97 100644
--- a/packages/mdx/lib/util/resolve-file-and-options.js
+++ b/packages/mdx/lib/util/resolve-file-and-options.js
@@ -12,7 +12,7 @@ import {md} from './extnames.js'
  * might contain `format: 'detect'`.
  *
  * @param {VFileCompatible} vfileCompatible
- * @param {CompileOptions} [options]
+ * @param {CompileOptions | null | undefined} [options]
  * @returns {{file: VFile, options: ProcessorOptions}}
  */
 export function resolveFileAndOptions(vfileCompatible, options) {
@@ -35,7 +35,7 @@ export function resolveFileAndOptions(vfileCompatible, options) {
 }
 
 /**
- * @param {VFileCompatible} [value]
+ * @param {VFileCompatible | null | undefined} [value]
  * @returns {value is VFile}
  */
 function looksLikeAVFile(value) {
diff --git a/packages/mdx/package.json b/packages/mdx/package.json
index 4e4f9aa3b..7848cd239 100644
--- a/packages/mdx/package.json
+++ b/packages/mdx/package.json
@@ -83,7 +83,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"^(compile|evaluate)\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/mdx/tsconfig.json b/packages/mdx/tsconfig.json
index 5fec52d2b..6cac7d0aa 100644
--- a/packages/mdx/tsconfig.json
+++ b/packages/mdx/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "*.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/node-loader/lib/index.js b/packages/node-loader/lib/index.js
index ccff13325..0dd9e09b6 100644
--- a/packages/node-loader/lib/index.js
+++ b/packages/node-loader/lib/index.js
@@ -2,13 +2,15 @@
  * @typedef {import('@mdx-js/mdx/lib/compile.js').CompileOptions} CompileOptions
  *
  * @typedef LoaderOptions
- * @property {boolean} [fixRuntimeWithoutExportMap=true]
+ *   Extra configuration.
+ * @property {boolean | null | undefined} [fixRuntimeWithoutExportMap=true]
  *   Several JSX runtimes, notably React below 18 and Emotion below 11.10.0,
  *   don’t yet have a proper export map set up.
  *   Export maps are needed to map `xxx/jsx-runtime` to an actual file in ESM.
  *   This option fixes React et al by turning those into `xxx/jsx-runtime.js`.
  *
  * @typedef {CompileOptions & LoaderOptions} Options
+ *   Configuration.
  */
 
 import fs from 'node:fs/promises'
@@ -20,11 +22,12 @@ import {createFormatAwareProcessors} from '@mdx-js/mdx/lib/util/create-format-aw
 /**
  * Create smart processors to handle different formats.
  *
- * @param {Options} [options]
+ * @param {Options | null | undefined} [options]
  */
-export function createLoader(options = {}) {
-  const {extnames, process} = createFormatAwareProcessors(options)
-  let fixRuntimeWithoutExportMap = options.fixRuntimeWithoutExportMap
+export function createLoader(options) {
+  const options_ = options || {}
+  const {extnames, process} = createFormatAwareProcessors(options_)
+  let fixRuntimeWithoutExportMap = options_.fixRuntimeWithoutExportMap
 
   if (
     fixRuntimeWithoutExportMap === null ||
diff --git a/packages/node-loader/package.json b/packages/node-loader/package.json
index 00085b9a5..fecc4fab2 100644
--- a/packages/node-loader/package.json
+++ b/packages/node-loader/package.json
@@ -48,7 +48,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "node --no-warnings --experimental-loader=./test/react-18-node-loader.js ../../node_modules/uvu/bin.js test \"\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/node-loader/tsconfig.json b/packages/node-loader/tsconfig.json
index 5fec52d2b..6cac7d0aa 100644
--- a/packages/node-loader/tsconfig.json
+++ b/packages/node-loader/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "*.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/preact/lib/index.js b/packages/preact/lib/index.js
index ee4c5fa7d..6c739dd29 100644
--- a/packages/preact/lib/index.js
+++ b/packages/preact/lib/index.js
@@ -4,14 +4,15 @@
  *
  * @typedef Props
  *   Configuration.
- * @property {Components} [components]
+ * @property {Components | MergeComponents | null | undefined} [components]
  *   Mapping of names for JSX components to Preact components.
- * @property {boolean} [disableParentContext=false]
+ * @property {boolean | null | undefined} [disableParentContext=false]
  *   Turn off outer component context.
- * @property {ComponentChildren} [children]
+ * @property {ComponentChildren | null | undefined} [children]
  *   Children.
  *
  * @callback MergeComponents
+ *   Custom merge function.
  * @param {Components} currentComponents
  *   Current components from the context.
  * @returns {Components}
@@ -34,7 +35,7 @@ import {useContext} from 'preact/hooks'
 export const MDXContext = createContext({})
 
 /**
- * @param {import('react').ComponentType<any>} Component
+ * @param {import('preact').ComponentType<any>} Component
  * @deprecated
  *   This export is marked as a legacy feature.
  *   That means it’s no longer recommended for use as it might be removed
@@ -46,12 +47,11 @@ export function withMDXComponents(Component) {
   return boundMDXComponent
 
   /**
-   * @param {Record<string, unknown> & {components?: Components}} props
+   * @param {Record<string, unknown> & {components?: Components | null | undefined}} props
    * @returns {JSX.Element}
    */
   function boundMDXComponent(props) {
     const allComponents = useMDXComponents(props.components)
-    // @ts-expect-error: React + Preact in this repo mess with TS.
     return h(Component, {...props, allComponents})
   }
 }
@@ -59,7 +59,7 @@ export function withMDXComponents(Component) {
 /**
  * Get current components from the MDX Context.
  *
- * @param {Components|MergeComponents} [components]
+ * @param {Components | MergeComponents | null | undefined} [components]
  *   Additional components to use or a function that takes the current
  *   components and filters/merges/changes them.
  * @returns {Components}
@@ -76,6 +76,9 @@ export function useMDXComponents(components) {
   return {...contextComponents, ...components}
 }
 
+/** @type {Components} */
+const emptyObject = {}
+
 /**
  * Provider for MDX context
  *
@@ -83,12 +86,21 @@ export function useMDXComponents(components) {
  * @returns {JSX.Element}
  */
 export function MDXProvider({components, children, disableParentContext}) {
-  let allComponents = useMDXComponents(components)
+  /** @type {Components} */
+  let allComponents
 
   if (disableParentContext) {
-    allComponents = components || {}
+    allComponents =
+      typeof components === 'function'
+        ? components({})
+        : components || emptyObject
+  } else {
+    allComponents = useMDXComponents(components)
   }
 
-  // @ts-expect-error: preact types are wrong.
-  return h(MDXContext.Provider, {value: allComponents}, children)
+  return h(
+    MDXContext.Provider,
+    {value: allComponents, children: undefined},
+    children
+  )
 }
diff --git a/packages/preact/package.json b/packages/preact/package.json
index 84f683abf..f20ccc7c7 100644
--- a/packages/preact/package.json
+++ b/packages/preact/package.json
@@ -54,7 +54,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"index.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "node --no-warnings --experimental-loader=../../script/jsx-loader.js ../../node_modules/uvu/bin.js test \"\\.jsx?$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/preact/test/test.jsx b/packages/preact/test/test.jsx
index 66dd10a83..83bcfd258 100644
--- a/packages/preact/test/test.jsx
+++ b/packages/preact/test/test.jsx
@@ -1,8 +1,11 @@
-/* @jsx h */
-/* @jsxFrag Fragment */
+/* @jsxRuntime automatic @jsxImportSource preact */
+
+// * @jsx h
+// * @jsxFrag Fragment
+// Note: this is unused by otherwise `xo` or so seems to fail?
+import {h} from 'preact'
 import {test} from 'uvu'
 import * as assert from 'uvu/assert'
-import {h} from 'preact'
 import * as runtime from 'preact/jsx-runtime'
 import {render} from 'preact-render-to-string'
 import {evaluate} from '@mdx-js/mdx'
@@ -18,7 +21,10 @@ test('should support `components` with `MDXProvider`', async () => {
     render(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <Content />
@@ -38,7 +44,9 @@ test('should support `wrapper` in `components`', async () => {
     render(
       <MDXProvider
         components={{
-          wrapper: (props) => <div id="layout" {...props} />
+          wrapper(/** @type {Record<string, unknown>} */ props) {
+            return <div id="layout" {...props} />
+          }
         }}
       >
         <Content />
@@ -58,13 +66,22 @@ test('should combine components in nested `MDXProvider`s', async () => {
     render(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />,
-          h2: (props) => <h2 style={{color: 'rebeccapurple'}} {...props} />
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          },
+          h2(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h2 style={{color: 'rebeccapurple'}} {...props} />
+          }
         }}
       >
         <MDXProvider
           components={{
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              // @ts-expect-error: Something wrong with Preact + React maybe?
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           }}
         >
           <Content />
@@ -85,13 +102,22 @@ test('should support components as a function', async () => {
     render(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />,
-          h2: (props) => <h2 style={{color: 'rebeccapurple'}} {...props} />
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          },
+          h2(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h2 style={{color: 'rebeccapurple'}} {...props} />
+          }
         }}
       >
         <MDXProvider
           components={() => ({
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              // @ts-expect-error: Something wrong with Preact + React maybe?
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           })}
         >
           <Content />
@@ -112,7 +138,10 @@ test('should support a `disableParentContext` prop (sandbox)', async () => {
     render(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <MDXProvider disableParentContext>
@@ -124,11 +153,46 @@ test('should support a `disableParentContext` prop (sandbox)', async () => {
   )
 })
 
+test('should support a `disableParentContext` *and* `components as a function', async () => {
+  const {default: Content} = await evaluate('# hi\n## hello', {
+    ...runtime,
+    useMDXComponents
+  })
+
+  assert.equal(
+    render(
+      <MDXProvider
+        components={{
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
+        }}
+      >
+        <MDXProvider
+          disableParentContext
+          components={() => ({
+            h2(props) {
+              // @ts-expect-error: Something wrong with Preact + React maybe?
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
+          })}
+        >
+          <Content />
+        </MDXProvider>
+      </MDXProvider>
+    ),
+    '<h1>hi</h1>\n<h2 style="color: papayawhip;">hello</h2>'
+  )
+})
+
 test('should support `withComponents`', async () => {
   const {default: Content} = await evaluate('# hi\n## hello', {
     ...runtime,
     useMDXComponents
   })
+  // Unknown props.
+  // type-coverage:ignore-next-line
   const With = withMDXComponents((props) => props.children)
 
   // Bug: this should use the `h2` component too, logically?
@@ -138,12 +202,18 @@ test('should support `withComponents`', async () => {
     render(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            // @ts-expect-error: Something wrong with Preact + React maybe?
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <With
           components={{
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              // @ts-expect-error: Something wrong with Preact + React maybe?
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           }}
         >
           <Content />
diff --git a/packages/preact/tsconfig.json b/packages/preact/tsconfig.json
index 94d319f4f..027bb5b04 100644
--- a/packages/preact/tsconfig.json
+++ b/packages/preact/tsconfig.json
@@ -1,4 +1,9 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "index.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"],
+  "compilerOptions": {
+    "jsx": "react-jsx",
+    "jsxImportSource": "preact"
+  }
 }
diff --git a/packages/react/lib/index.js b/packages/react/lib/index.js
index 421fc2d6a..3411d0957 100644
--- a/packages/react/lib/index.js
+++ b/packages/react/lib/index.js
@@ -4,14 +4,15 @@
  *
  * @typedef Props
  *   Configuration.
- * @property {Components} [components]
+ * @property {Components | MergeComponents | null | undefined} [components]
  *   Mapping of names for JSX components to React components.
- * @property {boolean} [disableParentContext=false]
+ * @property {boolean | null | undefined} [disableParentContext=false]
  *   Turn off outer component context.
- * @property {ReactNode} [children]
+ * @property {ReactNode | null | undefined} [children]
  *   Children.
  *
  * @callback MergeComponents
+ *   Custom merge function.
  * @param {Components} currentComponents
  *   Current components from the context.
  * @returns {Components}
@@ -45,7 +46,7 @@ export function withMDXComponents(Component) {
   return boundMDXComponent
 
   /**
-   * @param {Record<string, unknown> & {components?: Components}} props
+   * @param {Record<string, unknown> & {components?: Components | null | undefined}} props
    * @returns {JSX.Element}
    */
   function boundMDXComponent(props) {
@@ -57,7 +58,7 @@ export function withMDXComponents(Component) {
 /**
  * Get current components from the MDX Context.
  *
- * @param {Components|MergeComponents} [components]
+ * @param {Components | MergeComponents | null | undefined} [components]
  *   Additional components to use or a function that takes the current
  *   components and filters/merges/changes them.
  * @returns {Components}
@@ -65,6 +66,7 @@ export function withMDXComponents(Component) {
  */
 export function useMDXComponents(components) {
   const contextComponents = React.useContext(MDXContext)
+
   // Memoize to avoid unnecessary top-level context changes
   return React.useMemo(() => {
     // Custom merge via a function prop
@@ -86,10 +88,16 @@ const emptyObject = {}
  * @returns {JSX.Element}
  */
 export function MDXProvider({components, children, disableParentContext}) {
-  let allComponents = useMDXComponents(components)
+  /** @type {Components} */
+  let allComponents
 
   if (disableParentContext) {
-    allComponents = components || emptyObject
+    allComponents =
+      typeof components === 'function'
+        ? components({})
+        : components || emptyObject
+  } else {
+    allComponents = useMDXComponents(components)
   }
 
   return React.createElement(
diff --git a/packages/react/package.json b/packages/react/package.json
index b97450b45..51abe2f7f 100644
--- a/packages/react/package.json
+++ b/packages/react/package.json
@@ -56,7 +56,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"index.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "node --no-warnings --experimental-loader=../../script/jsx-loader.js ../../node_modules/uvu/bin.js test \"\\.jsx?$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/react/test/test.jsx b/packages/react/test/test.jsx
index a0107ab77..24f8a5256 100644
--- a/packages/react/test/test.jsx
+++ b/packages/react/test/test.jsx
@@ -1,11 +1,19 @@
+/**
+ * @typedef {import('react').ReactNode} ReactNode
+ */
+
 import {test} from 'uvu'
 import * as assert from 'uvu/assert'
 import {evaluate} from '@mdx-js/mdx'
 import React from 'react'
-import * as runtime from 'react/jsx-runtime'
+import * as runtimeRaw from 'react/jsx-runtime'
 import {renderToString} from 'react-dom/server'
 import {MDXProvider, useMDXComponents, withMDXComponents} from '../index.js'
 
+/** @type {{Fragment: unknown, jsx: unknown, jsxs: unknown}} */
+// @ts-expect-error: React types are wrong.
+const runtime = runtimeRaw
+
 test('should support `components` with `MDXProvider`', async () => {
   const {default: Content} = await evaluate('# hi', {
     ...runtime,
@@ -16,7 +24,9 @@ test('should support `components` with `MDXProvider`', async () => {
     renderToString(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <Content />
@@ -36,7 +46,9 @@ test('should support `wrapper` in `components`', async () => {
     renderToString(
       <MDXProvider
         components={{
-          wrapper: (props) => <div id="layout" {...props} />
+          wrapper(/** @type {Record<string, unknown>} */ props) {
+            return <div id="layout" {...props} />
+          }
         }}
       >
         <Content />
@@ -56,13 +68,19 @@ test('should combine components in nested `MDXProvider`s', async () => {
     renderToString(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />,
-          h2: (props) => <h2 style={{color: 'rebeccapurple'}} {...props} />
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          },
+          h2(props) {
+            return <h2 style={{color: 'rebeccapurple'}} {...props} />
+          }
         }}
       >
         <MDXProvider
           components={{
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           }}
         >
           <Content />
@@ -83,13 +101,19 @@ test('should support components as a function', async () => {
     renderToString(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />,
-          h2: (props) => <h2 style={{color: 'rebeccapurple'}} {...props} />
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          },
+          h2(props) {
+            return <h2 style={{color: 'rebeccapurple'}} {...props} />
+          }
         }}
       >
         <MDXProvider
           components={() => ({
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           })}
         >
           <Content />
@@ -110,7 +134,9 @@ test('should support a `disableParentContext` prop (sandbox)', async () => {
     renderToString(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <MDXProvider disableParentContext>
@@ -122,11 +148,44 @@ test('should support a `disableParentContext` prop (sandbox)', async () => {
   )
 })
 
+test('should support a `disableParentContext` *and* `components as a function', async () => {
+  const {default: Content} = await evaluate('# hi\n## hello', {
+    ...runtime,
+    useMDXComponents
+  })
+
+  assert.equal(
+    renderToString(
+      <MDXProvider
+        components={{
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
+        }}
+      >
+        <MDXProvider
+          disableParentContext
+          components={() => ({
+            h2(props) {
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
+          })}
+        >
+          <Content />
+        </MDXProvider>
+      </MDXProvider>
+    ),
+    '<h1>hi</h1>\n<h2 style="color:papayawhip">hello</h2>'
+  )
+})
+
 test('should support `withComponents`', async () => {
   const {default: Content} = await evaluate('# hi\n## hello', {
     ...runtime,
     useMDXComponents
   })
+  // Unknown props.
+  // type-coverage:ignore-next-line
   const With = withMDXComponents((props) => props.children)
 
   // Bug: this should use the `h2` component too, logically?
@@ -136,12 +195,16 @@ test('should support `withComponents`', async () => {
     renderToString(
       <MDXProvider
         components={{
-          h1: (props) => <h1 style={{color: 'tomato'}} {...props} />
+          h1(props) {
+            return <h1 style={{color: 'tomato'}} {...props} />
+          }
         }}
       >
         <With
           components={{
-            h2: (props) => <h2 style={{color: 'papayawhip'}} {...props} />
+            h2(props) {
+              return <h2 style={{color: 'papayawhip'}} {...props} />
+            }
           }}
         >
           <Content />
diff --git a/packages/react/tsconfig.json b/packages/react/tsconfig.json
index 94d319f4f..159a2649d 100644
--- a/packages/react/tsconfig.json
+++ b/packages/react/tsconfig.json
@@ -1,4 +1,8 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "index.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"],
+  "compilerOptions": {
+    "jsx": "preserve"
+  }
 }
diff --git a/packages/register/tsconfig.json b/packages/register/tsconfig.json
index e0270b1f1..6cac7d0aa 100644
--- a/packages/register/tsconfig.json
+++ b/packages/register/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.cjs", "test/**/*.cjs", "*.cjs"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/remark-mdx/index.js b/packages/remark-mdx/index.js
index 7517b3ceb..f56d5ab6a 100644
--- a/packages/remark-mdx/index.js
+++ b/packages/remark-mdx/index.js
@@ -2,9 +2,12 @@
  * @typedef {import('mdast').Root} Root
  * @typedef {import('micromark-extension-mdxjs').Options} MicromarkOptions
  * @typedef {import('mdast-util-mdx').ToMarkdownOptions} ToMarkdownOptions
+ * @typedef {import('mdast-util-mdx')} DoNotTouchAsThisIncludesMdxInTree
+ */
+
+/**
  * @typedef {MicromarkOptions & ToMarkdownOptions} Options
- *
- * @typedef {import('mdast-util-mdx')} DoNotTouchAsThisImportItIncludesMdxInTree
+ *   Configuration.
  */
 
 import {mdxjs} from 'micromark-extension-mdxjs'
@@ -15,9 +18,9 @@ import {mdxFromMarkdown, mdxToMarkdown} from 'mdast-util-mdx'
  * `{1 + 1}`; and JSX: `<Video id={123} />`).
  *
  * @this {import('unified').Processor}
- * @type {import('unified').Plugin<[Options?]|Array<void>, Root>}
+ * @type {import('unified').Plugin<[Options | null | undefined] | [], Root>}
  */
-export default function remarkMdx(options = {}) {
+export default function remarkMdx(options) {
   const data = this.data()
 
   add('micromarkExtensions', mdxjs(options))
diff --git a/packages/remark-mdx/package.json b/packages/remark-mdx/package.json
index ca1d47495..902af0dd0 100644
--- a/packages/remark-mdx/package.json
+++ b/packages/remark-mdx/package.json
@@ -53,7 +53,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"\\.jsx?$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/remark-mdx/test/index.js b/packages/remark-mdx/test/index.js
index 963a71116..fafbc9ac1 100644
--- a/packages/remark-mdx/test/index.js
+++ b/packages/remark-mdx/test/index.js
@@ -300,7 +300,7 @@ test('stringify', () => {
 })
 
 /**
- * @template {Root|Content} Tree
+ * @template {Root | Content} Tree
  * @param {Tree} tree
  * @returns {Tree}
  */
@@ -311,13 +311,13 @@ function clean(tree) {
 }
 
 /**
- * @param {Root|Content} tree
+ * @param {Root | Content} tree
  */
 function cleanEstree(tree) {
   visit(tree, onvisit)
 
   /**
-   * @param {Root|Content|MdxJsxAttribute|MdxJsxExpressionAttribute|MdxJsxAttributeValueExpression} node
+   * @param {Root | Content | MdxJsxAttribute | MdxJsxExpressionAttribute | MdxJsxAttributeValueExpression} node
    */
   function onvisit(node) {
     if (
diff --git a/packages/remark-mdx/tsconfig.json b/packages/remark-mdx/tsconfig.json
index 1c5f8be20..6cac7d0aa 100644
--- a/packages/remark-mdx/tsconfig.json
+++ b/packages/remark-mdx/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["test/**/*.js", "*.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/rollup/lib/index.js b/packages/rollup/lib/index.js
index 6e891ca3e..6f94e52f1 100644
--- a/packages/rollup/lib/index.js
+++ b/packages/rollup/lib/index.js
@@ -1,15 +1,22 @@
 /**
  * @typedef {import('@rollup/pluginutils').FilterPattern} FilterPattern
- * @typedef {Omit<import('@mdx-js/mdx').CompileOptions, 'SourceMapGenerator'>} CompileOptions
  * @typedef {import('rollup').Plugin} Plugin
+ * @typedef {import('rollup').SourceDescription} SourceDescription
+ */
+
+/**
+ * @typedef {Omit<import('@mdx-js/mdx').CompileOptions, 'SourceMapGenerator'>} CompileOptions
+ *   Default configuration.
  *
  * @typedef RollupPluginOptions
+ *   Extra configuration.
  * @property {FilterPattern} [include]
  *   List of picomatch patterns to include
  * @property {FilterPattern} [exclude]
  *   List of picomatch patterns to exclude
  *
  * @typedef {CompileOptions & RollupPluginOptions} Options
+ *   Configuration.
  */
 
 import {SourceMapGenerator} from 'source-map'
@@ -20,11 +27,13 @@ import {createFormatAwareProcessors} from '@mdx-js/mdx/lib/util/create-format-aw
 /**
  * Compile MDX w/ rollup.
  *
- * @param {Options} [options]
+ * @param {Options | null | undefined} [options]
+ *   Configuration.
  * @return {Plugin}
+ *   Rollup plugin.
  */
-export function rollup(options = {}) {
-  const {include, exclude, ...rest} = options
+export function rollup(options) {
+  const {include, exclude, ...rest} = options || {}
   const {extnames, process} = createFormatAwareProcessors({
     SourceMapGenerator,
     ...rest
@@ -42,7 +51,11 @@ export function rollup(options = {}) {
         extnames.includes(file.extname)
       ) {
         const compiled = await process(file)
-        return {code: String(compiled.value), map: compiled.map}
+        const code = String(compiled.value)
+        /** @type {SourceDescription} */
+        // @ts-expect-error: a) `undefined` is fine, b) `sourceRoot: undefined` is fine too.
+        const result = {code, map: compiled.map}
+        return result
         // V8 on Erbium.
         /* c8 ignore next 2 */
       }
diff --git a/packages/rollup/package.json b/packages/rollup/package.json
index 4c342910a..ed5df14aa 100644
--- a/packages/rollup/package.json
+++ b/packages/rollup/package.json
@@ -55,7 +55,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/rollup/tsconfig.json b/packages/rollup/tsconfig.json
index 5fec52d2b..6cac7d0aa 100644
--- a/packages/rollup/tsconfig.json
+++ b/packages/rollup/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "*.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/packages/vue/lib/index.js b/packages/vue/lib/index.js
index 1efeb5986..e075a29d6 100644
--- a/packages/vue/lib/index.js
+++ b/packages/vue/lib/index.js
@@ -4,7 +4,7 @@
 
 import {provide, inject, createVNode, Fragment} from 'vue'
 
-/** @type {import('vue').Component<{components?: Components}>} */
+/** @type {import('vue').Component<{components?: Components | null | undefined}>} */
 export const MDXProvider = {
   name: 'MDXProvider',
   props: {components: {type: Object, default: () => ({})}},
diff --git a/packages/vue/package.json b/packages/vue/package.json
index 9ee070a3f..daba24732 100644
--- a/packages/vue/package.json
+++ b/packages/vue/package.json
@@ -51,7 +51,7 @@
   },
   "scripts": {
     "prepack": "npm run build",
-    "build": "rimraf \"lib/**/*.d.ts\" \"test/**/*.d.ts\" \"index.d.ts\" && tsc && type-coverage",
+    "build": "tsc --build --clean && tsc --build && type-coverage",
     "test-api": "uvu test \"\\.js$\"",
     "test-coverage": "c8 --check-coverage --100 --reporter lcov npm run test-api",
     "test": "npm run build && npm run test-coverage"
diff --git a/packages/vue/test/test.js b/packages/vue/test/test.js
index 78211975b..f4a52e5a9 100644
--- a/packages/vue/test/test.js
+++ b/packages/vue/test/test.js
@@ -1,5 +1,4 @@
 /**
- * @typedef {import('mdx/types.js').MDXComponents} Components
  * @typedef {import('mdx/types.js').MDXContent} MDXContent
  * @typedef {import('mdx/types.js').MDXModule} MDXModule
  * @typedef {import('vue').Component} AnyComponent
@@ -39,7 +38,7 @@ async function evaluate(value) {
 
 /**
  * @param {AnyComponent} root
- * @param {Record<string, unknown>} [rootProps]
+ * @param {Record<string, unknown> | null | undefined} [rootProps]
  * @returns {Promise<string>}
  */
 async function vueToString(root, rootProps) {
diff --git a/packages/vue/tsconfig.json b/packages/vue/tsconfig.json
index 94d319f4f..6cac7d0aa 100644
--- a/packages/vue/tsconfig.json
+++ b/packages/vue/tsconfig.json
@@ -1,4 +1,5 @@
 {
   "extends": "../../tsconfig.json",
-  "include": ["lib/**/*.js", "test/**/*.js", "index.js"]
+  "include": ["**/*.cjs", "**/*.js", "**/*.jsx"],
+  "exclude": ["coverage/", "node_modules/"]
 }
diff --git a/script/jsx-loader.js b/script/jsx-loader.js
index 8a603e680..757cc9272 100644
--- a/script/jsx-loader.js
+++ b/script/jsx-loader.js
@@ -37,8 +37,11 @@ export function createLoader() {
 
     if (warnings && warnings.length > 0) {
       for (const warning of warnings) {
-        console.log(warning.location)
-        console.log(warning.text)
+        console.log(
+          'script/jsx-loader.js: warning: %j: %s',
+          warning.location,
+          warning.text
+        )
       }
     }
 
diff --git a/tsconfig.json b/tsconfig.json
index d06db7dc4..8d833a181 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,12 +1,15 @@
 {
   "compilerOptions": {
-    "target": "es2021",
-    "lib": ["es2021"],
-    "module": "node16",
     "checkJs": true,
     "declaration": true,
     "emitDeclarationOnly": true,
+    "exactOptionalPropertyTypes": true,
+    "forceConsistentCasingInFileNames": true,
+    "lib": ["es2021"],
+    "module": "node16",
+    "newLine": "lf",
     "skipLibCheck": true,
-    "strict": true
+    "strict": true,
+    "target": "es2021"
   }
 }