diff --git a/.gitignore b/.gitignore
index 735f4af..c977c85 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 .DS_Store
+*.d.ts
 *.log
 coverage/
 node_modules/
diff --git a/index.js b/index.js
index 6f42cf3..8107bdf 100644
--- a/index.js
+++ b/index.js
@@ -1,34 +1,58 @@
+/**
+ * @typedef {import('unist').Node} Node
+ * @typedef {import('unist').Parent} Parent
+ *
+ * @typedef {import('unist-util-is').Type} Type
+ * @typedef {import('unist-util-is').Props} Props
+ * @typedef {import('unist-util-is').TestFunctionAnything} TestFunctionAnything
+ */
+
 import {convert} from 'unist-util-is'
 
-export function findBefore(parent, index, test) {
-  var is = convert(test)
+export var findBefore =
+  /**
+   * @type {(
+   *  (<T extends Node>(node: Parent, index: Node|number, test: T['type']|Partial<T>|import('unist-util-is').TestFunctionPredicate<T>|Array.<T['type']|Partial<T>|import('unist-util-is').TestFunctionPredicate<T>>) => T|null) &
+   *  ((node: Parent, index: Node|number, test?: null|undefined|Type|Props|TestFunctionAnything|Array<Type|Props|TestFunctionAnything>) => Node|null)
+   * )}
+   */
+  (
+    /**
+     * @param {Parent} parent Parent node
+     * @param {Node|number} index Child of `parent`, or it’s index
+     * @param {null|undefined|Type|Props|TestFunctionAnything|Array<Type|Props|TestFunctionAnything>} [test] is-compatible test (such as a type)
+     * @returns {Node|null}
+     */
+    function (parent, index, test) {
+      var is = convert(test)
 
-  if (!parent || !parent.type || !parent.children) {
-    throw new Error('Expected parent node')
-  }
+      if (!parent || !parent.type || !parent.children) {
+        throw new Error('Expected parent node')
+      }
 
-  if (typeof index === 'number') {
-    if (index < 0 || index === Number.POSITIVE_INFINITY) {
-      throw new Error('Expected positive finite number as index')
-    }
-  } else {
-    index = parent.children.indexOf(index)
+      if (typeof index === 'number') {
+        if (index < 0 || index === Number.POSITIVE_INFINITY) {
+          throw new Error('Expected positive finite number as index')
+        }
+      } else {
+        index = parent.children.indexOf(index)
 
-    if (index < 0) {
-      throw new Error('Expected child node or index')
-    }
-  }
+        if (index < 0) {
+          throw new Error('Expected child node or index')
+        }
+      }
 
-  // Performance.
-  if (index > parent.children.length) {
-    index = parent.children.length
-  }
+      // Performance.
+      if (index > parent.children.length) {
+        index = parent.children.length
+      }
 
-  while (index--) {
-    if (is(parent.children[index], index, parent)) {
-      return parent.children[index]
-    }
-  }
+      while (index--) {
+        if (is(parent.children[index], index, parent)) {
+          return parent.children[index]
+        }
+      }
 
-  return null
-}
+      return null
+    }
+  )
diff --git a/package.json b/package.json
index a2e8118..d86ca83 100644
--- a/package.json
+++ b/package.json
@@ -25,26 +25,35 @@
   "sideEffects": false,
   "type": "module",
   "main": "index.js",
+  "types": "index.d.ts",
   "files": [
+    "index.d.ts",
     "index.js"
   ],
   "dependencies": {
+    "@types/unist": "^2.0.0",
     "unist-util-is": "^5.0.0"
   },
   "devDependencies": {
+    "@types/tape": "^4.0.0",
     "c8": "^7.0.0",
     "prettier": "^2.0.0",
     "remark": "^13.0.0",
     "remark-cli": "^9.0.0",
     "remark-preset-wooorm": "^8.0.0",
+    "rimraf": "^3.0.0",
     "tape": "^5.0.0",
+    "type-coverage": "^2.0.0",
+    "typescript": "^4.0.0",
     "xo": "^0.38.0"
   },
   "scripts": {
+    "prepack": "npm run build && npm run format",
+    "build": "rimraf \"*.d.ts\" && tsc && type-coverage",
     "format": "remark . -qfo && prettier . -w --loglevel warn && xo --fix",
     "test-api": "node test.js",
     "test-coverage": "c8 --check-coverage --branches 100 --functions 100 --lines 100 --statements 100 --reporter lcov node test.js",
-    "test": "npm run format && npm run test-coverage"
+    "test": "npm run build && npm run format && npm run test-coverage"
   },
   "prettier": {
     "tabWidth": 2,
@@ -57,6 +66,7 @@
   "xo": {
     "prettier": true,
     "rules": {
+      "import/no-mutable-exports": "off",
       "no-var": "off",
       "prefer-arrow-callback": "off"
     }
@@ -65,5 +75,10 @@
     "plugins": [
       "preset-wooorm"
     ]
+  },
+  "typeCoverage": {
+    "atLeast": 100,
+    "detail": true,
+    "strict": true
   }
 }
diff --git a/test.js b/test.js
index bc4c24c..3af1755 100644
--- a/test.js
+++ b/test.js
@@ -1,3 +1,7 @@
+/**
+ * @typedef {import('unist').Node} Node
+ */
+
 import test from 'tape'
 import remark from 'remark'
 import {findBefore} from './index.js'
@@ -9,6 +13,7 @@ var children = paragraph.children
 test('unist-util-find-before', function (t) {
   t.throws(
     function () {
+      // @ts-ignore runtime
       findBefore()
     },
     /Expected parent node/,
@@ -17,9 +22,8 @@ test('unist-util-find-before', function (t) {
 
   t.throws(
     function () {
-      findBefore({
-        type: 'foo'
-      })
+      // @ts-ignore runtime
+      findBefore({type: 'foo'})
     },
     /Expected parent node/,
     'should fail without parent node'
@@ -27,6 +31,7 @@ test('unist-util-find-before', function (t) {
 
   t.throws(
     function () {
+      // @ts-ignore runtime
       findBefore({type: 'foo', children: []})
     },
     /Expected child node or index/,
@@ -51,6 +56,7 @@ test('unist-util-find-before', function (t) {
 
   t.throws(
     function () {
+      // @ts-ignore runtime
       findBefore({type: 'foo', children: [{type: 'bar'}]}, 1, false)
     },
     /Expected function, string, or object as test/,
@@ -59,6 +65,7 @@ test('unist-util-find-before', function (t) {
 
   t.throws(
     function () {
+      // @ts-ignore runtime
       findBefore({type: 'foo', children: [{type: 'bar'}]}, 1, true)
     },
     /Expected function, string, or object as test/,
@@ -154,7 +161,11 @@ test('unist-util-find-before', function (t) {
     'should return a child when given a `test` and existing (#4)'
   )
 
-  function test(node, n) {
+  /**
+   * @param {Node} _
+   * @param {number} n
+   */
+  function test(_, n) {
     return n === 3
   }
 
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..be08abe
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "include": ["*.js"],
+  "compilerOptions": {
+    "target": "ES2020",
+    "lib": ["ES2020"],
+    "module": "ES2020",
+    "moduleResolution": "node",
+    "allowJs": true,
+    "checkJs": true,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "allowSyntheticDefaultImports": true,
+    "skipLibCheck": true
+  }
+}