diff --git a/.gitignore b/.gitignore
index 33d4929..53a29e3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,5 +1,6 @@
 coverage/
 node_modules/
 .DS_Store
+*.d.ts
 *.log
 yarn.lock
diff --git a/cli.js b/cli.js
index ac19ba7..214946b 100755
--- a/cli.js
+++ b/cli.js
@@ -1,6 +1,7 @@
 #!/usr/bin/env node
 import fs from 'node:fs'
 import process from 'node:process'
+import {URL} from 'node:url'
 import notifier from 'update-notifier'
 import supportsColor from 'supports-color'
 import meow from 'meow'
@@ -20,8 +21,9 @@ import retextProfanities from 'retext-profanities'
 import unifiedDiff from 'unified-diff'
 import {filter} from './filter.js'
 
+/** @type {import('type-fest').PackageJson} */
 const pack = JSON.parse(
-  fs.readFileSync(new URL('./package.json', import.meta.url))
+  String(fs.readFileSync(new URL('./package.json', import.meta.url)))
 )
 
 const textExtensions = [
@@ -38,6 +40,7 @@ const htmlExtensions = ['htm', 'html']
 const mdxExtensions = ['mdx']
 
 // Update messages.
+/** @ts-expect-error: `package.json` is fine. */
 notifier({pkg: pack}).notify()
 
 // Set-up meow.
@@ -88,7 +91,9 @@ const extensions = cli.flags.html
   ? mdxExtensions
   : textExtensions
 const defaultGlobs = ['{docs/**/,doc/**/,}*.{' + extensions.join(',') + '}']
+/** @type {boolean|undefined} */
 let silentlyIgnore
+/** @type {string[]|undefined} */
 let globs
 
 if (cli.flags.stdin) {
@@ -112,7 +117,7 @@ engine(
     output: false,
     rcName: '.alexrc',
     packageField: 'alex',
-    color: supportsColor.stderr,
+    color: Boolean(supportsColor.stderr),
     reporter: cli.flags.reporter || vfileReporter,
     reporterOptions: {
       verbose: cli.flags.why
@@ -121,7 +126,7 @@ engine(
     ignoreName: '.alexignore',
     silentlyIgnore,
     frail: true,
-    defaultConfig: transform()
+    defaultConfig: transform({})
   },
   function (error, code) {
     if (error) console.error(error.message)
@@ -129,28 +134,35 @@ engine(
   }
 )
 
-function transform(options) {
-  const settings = options || {}
+/**
+ * @type {import('unified-engine').ConfigTransform}
+ * @param {import('./index.js').OptionsObject} [options]
+ */
+function transform(options = {}) {
+  /** @type {import('unified').PluggableList} */
   let plugins = [
     retextEnglish,
-    [retextProfanities, {sureness: settings.profanitySureness}],
-    [retextEquality, {noBinary: settings.noBinary}]
+    [retextProfanities, {sureness: options.profanitySureness}],
+    [retextEquality, {noBinary: options.noBinary}]
   ]
 
   if (cli.flags.html) {
+    // @ts-expect-error: types are having a hard time for bridges.
     plugins = [rehypeParse, [rehypeRetext, unified().use({plugins})]]
   } else if (cli.flags.mdx) {
+    // @ts-expect-error: types are having a hard time for bridges.
     plugins = [remarkParse, remarkMdx, [remarkRetext, unified().use({plugins})]]
   } else if (!cli.flags.text) {
     plugins = [
       remarkParse,
       remarkGfm,
       [remarkFrontmatter, ['yaml', 'toml']],
+      // @ts-expect-error: types are having a hard time for bridges.
       [remarkRetext, unified().use({plugins})]
     ]
   }
 
-  plugins.push([filter, {allow: settings.allow, deny: settings.deny}])
+  plugins.push([filter, {allow: options.allow, deny: options.deny}])
 
   // Hard to check.
   /* c8 ignore next 3 */
diff --git a/filter.js b/filter.js
index f04be92..9671fb4 100644
--- a/filter.js
+++ b/filter.js
@@ -1,10 +1,25 @@
-import remarkMessageControl from 'remark-message-control'
+/**
+ * @typedef {import('mdast').Root} Root
+ *
+ * @typedef Options
+ *   Configuration.
+ * @property {string[]} [deny]
+ *   The `deny` field should be an array of rules or `undefined` (the default is
+ *   `undefined`).
+ *   When provided, *only* the rules specified are reported.
+ *   You cannot use both `allow` and `deny` at the same time.
+ * @property {string[]} [allow]
+ *   The `allow` field should be an array of rules or `undefined` (the default
+ *   is `undefined`).
+ *   When provided, the rules specified are skipped and not reported.
+ *   You cannot use both `allow` and `deny` at the same time.
+ */
 
-export function filter(options) {
-  /* c8 ignore next */
-  const settings = options || {}
+import remarkMessageControl from 'remark-message-control'
 
-  if (settings.allow && settings.deny) {
+/** @type {import('unified').Plugin<[Options?]|[], Root>} */
+export function filter(options = {}) {
+  if (options.allow && options.deny) {
     throw new Error(
       'Do not provide both allow and deny configuration parameters'
     )
@@ -12,9 +27,9 @@ export function filter(options) {
 
   return remarkMessageControl({
     name: 'alex',
-    reset: Boolean(settings.deny),
-    enable: settings.deny,
-    disable: settings.allow,
+    reset: Boolean(options.deny),
+    enable: options.deny,
+    disable: options.allow,
     source: ['retext-equality', 'retext-profanities']
   })
 }
diff --git a/index.js b/index.js
index 13dad05..c04c5a0 100644
--- a/index.js
+++ b/index.js
@@ -1,3 +1,17 @@
+/**
+ * @typedef {import('nlcst').Root} Root
+ * @typedef {import('./filter.js').Options} FilterOptions
+ *
+ * @typedef {boolean|undefined} NoBinaryOption
+ * @typedef {0|1|2|undefined} SurenessOption
+ *
+ * @typedef {{noBinary: NoBinaryOption, sureness: SurenessOption}} TextOptions
+ *
+ * @typedef {{noBinary?: NoBinaryOption, profanitySureness?: SurenessOption} & FilterOptions} OptionsObject
+ * @typedef {import('vfile').VFileCompatible} Input
+ * @typedef {OptionsObject|string[]|undefined} Options
+ */
+
 import {VFile} from 'vfile'
 import {unified} from 'unified'
 import remarkParse from 'remark-parse'
@@ -13,31 +27,24 @@ import rehypeRetext from 'rehype-retext'
 import {sort} from 'vfile-sort'
 import {filter} from './filter.js'
 
-function makeText(config) {
+/** @param {TextOptions} options */
+function makeText(options) {
   return unified()
     .use(retextEnglish)
-    .use(retextEquality, {
-      noBinary: config && config.noBinary
-    })
-    .use(retextProfanities, {
-      sureness: config && config.profanitySureness
-    })
+    .use(retextEquality, options)
+    .use(retextProfanities, options)
 }
 
-// Alex’s core.
-function core(value, config, processor) {
-  let allow
-  let deny
-
-  if (Array.isArray(config)) {
-    allow = config
-  } else if (config) {
-    allow = config.allow
-    deny = config.deny
-  }
-
+/**
+ * Alex’s core.
+ *
+ * @param {Input} value
+ * @param {FilterOptions} options
+ * @param {import('unified').Processor<void, Root>} processor
+ */
+function core(value, options, processor) {
   const file = new VFile(value)
-  const tree = processor.use(filter, {allow, deny}).parse(file)
+  const tree = processor.use(filter, options).parse(file)
 
   processor.runSync(tree, file)
 
@@ -48,41 +55,90 @@ function core(value, config, processor) {
 
 export default markdown
 
-// Alex.
+/**
+ * Alex (markdown).
+ *
+ * @param {Input} value
+ * @param {Options} [config]
+ */
 export function markdown(value, config) {
+  const options = splitOptions(config)
   return core(
     value,
-    config,
+    options.filter,
     unified()
       .use(remarkParse)
       .use(remarkGfm)
       .use(remarkFrontmatter, ['yaml', 'toml'])
-      .use(remarkRetext, makeText(config))
+      .use(remarkRetext, makeText(options.text))
   )
 }
 
-// Alex, for MDX.
+/**
+ * Alex (MDX).
+ *
+ * @param {Input} value
+ * @param {Options} [config]
+ */
 export function mdx(value, config) {
+  const options = splitOptions(config)
   return core(
     value,
-    config,
+    options.filter,
     unified()
       .use(remarkParse)
       .use(remarkMdx)
-      .use(remarkRetext, makeText(config))
+      .use(remarkRetext, makeText(options.text))
   )
 }
 
-// Alex, for HTML.
+/**
+ * Alex (HTML).
+ *
+ * @param {Input} value
+ * @param {Options} [config]
+ */
 export function html(value, config) {
+  const options = splitOptions(config)
   return core(
     value,
-    config,
-    unified().use(rehypeParse).use(rehypeRetext, makeText(config))
+    options.filter,
+    unified().use(rehypeParse).use(rehypeRetext, makeText(options.text))
   )
 }
 
-// Alex, without the markdown.
+/**
+ * Alex (plain text).
+ *
+ * @param {Input} value
+ * @param {Options} [config]
+ */
 export function text(value, config) {
-  return core(value, config, makeText(config))
+  const options = splitOptions(config)
+  return core(value, options.filter, makeText(options.text))
+}
+
+/**
+ * @param {Options} options
+ */
+function splitOptions(options) {
+  /** @type {string[]|undefined} */
+  let allow
+  /** @type {string[]|undefined} */
+  let deny
+  /** @type {boolean|undefined} */
+  let noBinary
+  /** @type {SurenessOption} */
+  let sureness
+
+  if (Array.isArray(options)) {
+    allow = options
+  } else if (options) {
+    allow = options.allow
+    deny = options.deny
+    noBinary = options.noBinary
+    sureness = options.profanitySureness
+  }
+
+  return {filter: {allow, deny}, text: {noBinary, sureness}}
 }
diff --git a/package.json b/package.json
index 96f1261..f20de54 100644
--- a/package.json
+++ b/package.json
@@ -58,12 +58,17 @@
   "type": "module",
   "sideEffects": false,
   "bin": "cli.js",
+  "types": "index.d.ts",
   "files": [
+    "index.d.ts",
     "index.js",
+    "filter.d.ts",
     "filter.js",
     "cli.js"
   ],
   "dependencies": {
+    "@types/mdast": "^3.0.0",
+    "@types/nlcst": "^1.0.0",
     "meow": "^10.0.0",
     "rehype-parse": "^8.0.0",
     "rehype-retext": "^3.0.0",
@@ -85,19 +90,26 @@
     "vfile-sort": "^3.0.0"
   },
   "devDependencies": {
+    "@types/tape": "^4.0.0",
+    "@types/update-notifier": "^5.0.0",
     "c8": "^7.10.0",
     "prettier": "^2.0.0",
     "remark-cli": "^10.0.0",
     "remark-preset-wooorm": "^9.0.0",
+    "rimraf": "^3.0.0",
     "tape": "^5.0.0",
+    "type-coverage": "^2.0.0",
+    "type-fest": "^2.0.0",
+    "typescript": "^4.0.0",
     "vfile-reporter-json": "^3.0.0",
     "xo": "^0.45.0"
   },
   "scripts": {
+    "build": "rimraf \"test/**/*.d.ts\" \"*.d.ts\" && tsc && type-coverage",
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node test/index.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov npm run test-api",
-    "test": "npm run format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "alex": {
     "allow": [
@@ -129,5 +141,11 @@
         }
       ]
     ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true,
+    "ignoreCatch": true
   }
 }
diff --git a/test/api.js b/test/api.js
index 5c13560..e3cfbb5 100644
--- a/test/api.js
+++ b/test/api.js
@@ -1,4 +1,5 @@
 import fs from 'node:fs'
+import {URL} from 'node:url'
 import test from 'tape'
 import alex, {markdown, mdx, text, html} from '../index.js'
 
diff --git a/test/cli.js b/test/cli.js
index c14f538..5650c11 100644
--- a/test/cli.js
+++ b/test/cli.js
@@ -4,65 +4,63 @@ import path from 'node:path'
 import process from 'node:process'
 import test from 'tape'
 
-const pkg = JSON.parse(fs.readFileSync('package.json'))
+/** @type {import('type-fest').PackageJson} */
+const pkg = JSON.parse(String(fs.readFileSync('package.json')))
 
 test('alex-cli', function (t) {
   t.test('version', function (t) {
     t.plan(1)
 
-    childProcess.exec('./cli.js -v', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js -v', (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, '', pkg.version + '\n'],
         'should work'
       )
-    }
+    })
   })
 
   t.test('help', function (t) {
     t.plan(1)
 
-    childProcess.exec('./cli.js -h', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js -h', (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, /Usage: alex \[<glob> ...] /.test(stdout)],
         [null, '', true],
         'should work'
       )
-    }
+    })
   })
 
   t.test('stdin', function (t) {
     t.plan(1)
 
-    const subprocess = childProcess.exec('./cli.js --stdin', onexec)
-
-    setTimeout(end, 10)
-
-    function end() {
-      subprocess.stdin.end('His')
-    }
-
-    function onexec(error, stdout, stderr) {
-      t.deepEqual(
-        [error.code, stderr, stdout],
-        [
-          1,
+    const subprocess = childProcess.exec(
+      './cli.js --stdin',
+      (error, stdout, stderr) => {
+        t.deepEqual(
+          [error && error.code, stderr, stdout],
           [
-            '<stdin>',
-            '  1:1-1:4  warning  `His` may be insensitive, when referring to a person, use `Their`, `Theirs`, `Them` instead  her-him  retext-equality',
-            '',
-            '⚠ 1 warning',
+            1,
+            [
+              '<stdin>',
+              '  1:1-1:4  warning  `His` may be insensitive, when referring to a person, use `Their`, `Theirs`, `Them` instead  her-him  retext-equality',
+              '',
+              '⚠ 1 warning',
+              ''
+            ].join('\n'),
             ''
-          ].join('\n'),
-          ''
-        ],
-        'should work'
-      )
-    }
+          ],
+          'should work'
+        )
+      }
+    )
+
+    setTimeout(function () {
+      if (subprocess.stdin) {
+        subprocess.stdin.end('His')
+      }
+    }, 10)
   })
 
   t.test('stdin and globs', function (t) {
@@ -70,15 +68,17 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js --stdin ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js --stdin ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, /Do not pass globs with `--stdin`/.test(stderr), stdout],
+        [
+          error && error.code,
+          /Do not pass globs with `--stdin`/.test(stderr),
+          stdout
+        ],
         [1, true, ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('markdown by default', function (t) {
@@ -86,15 +86,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, fp + ': no issues found\n', ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('optionally html', function (t) {
@@ -102,15 +100,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' --html', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp + ' --html', (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, /3 warnings/.test(stderr), stdout],
+        [error && error.code, /3 warnings/.test(stderr), stdout],
         [1, true, ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('optionally text (on markdown)', function (t) {
@@ -118,15 +114,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' --text', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp + ' --text', (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, /1 warning/.test(stderr), stdout],
+        [error && error.code, /1 warning/.test(stderr), stdout],
         [1, true, ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('optionally text (on html)', function (t) {
@@ -134,15 +128,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' --text', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp + ' --text', (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, /10 warnings/.test(stderr), stdout],
+        [error && error.code, /10 warnings/.test(stderr), stdout],
         [1, true, ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('mdx', function (t) {
@@ -150,15 +142,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' --mdx', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp + ' --mdx', (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, /2 warnings/.test(stderr), stdout],
+        [error && error.code, /2 warnings/.test(stderr), stdout],
         [1, true, ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('successful', function (t) {
@@ -166,15 +156,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, fp + ': no issues found\n', ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('quiet (ok)', function (t) {
@@ -182,11 +170,9 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' -q', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp + ' -q', (error, stdout, stderr) => {
       t.deepEqual([error, stderr, stdout], [null, '', ''], 'should work')
-    }
+    })
   })
 
   t.test('quiet (on error)', function (t) {
@@ -194,25 +180,26 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp + ' -q --text', onexec)
-
-    function onexec(error, stdout, stderr) {
-      t.deepEqual(
-        [error.code, stderr, stdout],
-        [
-          1,
+    childProcess.exec(
+      './cli.js ' + fp + ' -q --text',
+      (error, stdout, stderr) => {
+        t.deepEqual(
+          [error && error.code, stderr, stdout],
           [
-            fp,
-            '  1:18-1:21  warning  `his` may be insensitive, when referring to a person, use `their`, `theirs`, `them` instead  her-him  retext-equality',
-            '',
-            '⚠ 1 warning',
+            1,
+            [
+              fp,
+              '  1:18-1:21  warning  `his` may be insensitive, when referring to a person, use `their`, `theirs`, `them` instead  her-him  retext-equality',
+              '',
+              '⚠ 1 warning',
+              ''
+            ].join('\n'),
             ''
-          ].join('\n'),
-          ''
-        ],
-        'should work'
-      )
-    }
+          ],
+          'should work'
+        )
+      }
+    )
   })
 
   t.test('binary (default: ok)', function (t) {
@@ -220,15 +207,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, fp + ': no issues found\n', ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('binary (with config file)', function (t) {
@@ -236,11 +221,9 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, stderr, stdout],
+        [error && error.code, stderr, stdout],
         [
           1,
           [
@@ -255,7 +238,7 @@ test('alex-cli', function (t) {
         ],
         'should work'
       )
-    }
+    })
   })
 
   t.test('profanity (default)', function (t) {
@@ -263,11 +246,9 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, stderr, stdout],
+        [error && error.code, stderr, stdout],
         [
           1,
           [
@@ -281,7 +262,7 @@ test('alex-cli', function (t) {
         ],
         'should work'
       )
-    }
+    })
   })
 
   t.test('profanity (with config file)', function (t) {
@@ -289,15 +270,13 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, fp + ': no issues found\n', ''],
         'should work'
       )
-    }
+    })
   })
 
   t.test('custom reporter', function (t) {
@@ -305,7 +284,16 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js --reporter json ' + fp, onexec)
+    childProcess.exec(
+      './cli.js --reporter json ' + fp,
+      (error, stdout, stderr) => {
+        t.deepEqual(
+          [error, stdout, stderr],
+          [null, '', expectedJson + '\n'],
+          'should work'
+        )
+      }
+    )
 
     const expectedJson = JSON.stringify([
       {
@@ -315,14 +303,6 @@ test('alex-cli', function (t) {
         messages: []
       }
     ])
-
-    function onexec(error, stdout, stderr) {
-      t.deepEqual(
-        [error, stdout, stderr],
-        [null, '', expectedJson + '\n'],
-        'should work'
-      )
-    }
   })
 
   t.test("custom formatter that isn't installed", function (t) {
@@ -330,15 +310,16 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js --reporter doesntexist ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
-      t.deepEqual(
-        [error, stderr, stdout],
-        [null, 'Could not find reporter `doesntexist`\n', ''],
-        'should work'
-      )
-    }
+    childProcess.exec(
+      './cli.js --reporter doesntexist ' + fp,
+      (error, stdout, stderr) => {
+        t.deepEqual(
+          [error, stderr, stdout],
+          [null, 'Could not find reporter `doesntexist`\n', ''],
+          'should work'
+        )
+      }
+    )
   })
 
   t.test('deny', function (t) {
@@ -346,11 +327,9 @@ test('alex-cli', function (t) {
 
     t.plan(1)
 
-    childProcess.exec('./cli.js ' + fp, onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js ' + fp, (error, stdout, stderr) => {
       t.deepEqual(
-        [error.code, stderr, stdout],
+        [error && error.code, stderr, stdout],
         [
           1,
           [
@@ -364,21 +343,19 @@ test('alex-cli', function (t) {
         ],
         'should work'
       )
-    }
+    })
   })
 
   t.test('default globs', function (t) {
     t.plan(1)
 
-    childProcess.exec('./cli.js', onexec)
-
-    function onexec(error, stdout, stderr) {
+    childProcess.exec('./cli.js', (error, stdout, stderr) => {
       t.deepEqual(
         [error, stderr, stdout],
         [null, 'readme.md: no issues found\n', ''],
         'should work'
       )
-    }
+    })
   })
 
   t.end()
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..a93b9f9
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "include": ["test/**/*.js", "*.js"],
+  "compilerOptions": {
+    "target": "ES2020",
+    "lib": ["ES2020"],
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "allowJs": true,
+    "checkJs": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true,
+    "strict": true
+  }
+}