diff --git a/.github/workflows/dev.yml b/.github/workflows/dev.yml
index 99279de..703bf8d 100644
--- a/.github/workflows/dev.yml
+++ b/.github/workflows/dev.yml
@@ -45,6 +45,9 @@ jobs:
       - name: Check format
         run: npm run format-check
 
+      - name: Check types
+        run: npm run types-check
+
       - name: Coveralls
         uses: coverallsapp/github-action@8d6379e14d29928660c4ba802d8e85393440b329 # v2.3.8
         with:
diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..e3f9081
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,56 @@
+/**
+ * Find a file given configuration and defaults describing lookup behavior.
+ *
+ * @param {LookupConfig | string} [pathObj] The user-provided configuration for file lookup.
+ * @param {LookupConfig} [defaultObj] Default configuration for file lookup.
+ * @returns {Found | null} Information about the found file or `null` if it cannot be found.
+ */
+export default function fined(
+  pathObj?: LookupConfig | string,
+  defaultObj?: LookupConfig,
+): Found | null;
+/**
+ * Object mapping of extensions to some value.
+ */
+export type ExtensionMap = {
+  [extension: string]: any;
+};
+/**
+ * Configuration used to lookup a file.
+ */
+export type LookupConfig = {
+  /**
+   * The base path to search within.
+   */
+  path?: string | null | undefined;
+  /**
+   * The name of the file without an extension.
+   */
+  name?: string | null | undefined;
+  /**
+   * The extensions the file being looked up may have.
+   */
+  extensions?: string | string[] | ExtensionMap | null | undefined;
+  /**
+   * The current working directory that `path + name` is resolved against.
+   */
+  cwd?: string | null | undefined;
+  /**
+   * Whether file lookup should recurse up the directory structure.
+   */
+  findUp?: boolean | null | undefined;
+};
+/**
+ * Information about the found file.
+ */
+export type Found = {
+  /**
+   * The absolute path of the found file.
+   */
+  path: string;
+  /**
+   * The extension that matched from configured extensions.
+   */
+  extension: string | ExtensionMap;
+};
+//# sourceMappingURL=index.d.ts.map
diff --git a/index.d.ts.map b/index.d.ts.map
new file mode 100644
index 0000000..0cd518f
--- /dev/null
+++ b/index.d.ts.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.d.ts","sourceRoot":"","sources":["index.js"],"names":[],"mappings":"AAKA;;;;;;GAMG;AACH,wCAJW,YAAY,GAAG,MAAM,eACrB,YAAY,GACV,KAAK,GAAG,IAAI,CAKxB;;;;2BAGY;IAAE,CAAC,SAAS,EAAE,MAAM,GAAG,GAAC,CAAA;CAAE;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;UAczB,MAAM;;;;eACN,MAAM,GAAG,YAAY"}
\ No newline at end of file
diff --git a/index.js b/index.js
index af3cc1d..22e188a 100644
--- a/index.js
+++ b/index.js
@@ -1,12 +1,46 @@
 import os from "node:os";
 import fs from "node:fs";
 import path from "node:path";
-
+import process from "node:process";
+
+/**
+ * Find a file given configuration and defaults describing lookup behavior.
+ *
+ * @param {LookupConfig | string} [pathObj] The user-provided configuration for file lookup.
+ * @param {LookupConfig} [defaultObj] Default configuration for file lookup.
+ * @returns {Found | null} Information about the found file or `null` if it cannot be found.
+ */
 export default function fined(pathObj, defaultObj) {
   const expandedPath = expandPath(pathObj, defaultObj);
   return expandedPath ? findWithExpandedPath(expandedPath) : null;
 }
 
+/**
+ * @typedef {{ [extension: string]: * }} ExtensionMap Object mapping of extensions to some value.
+ */
+
+/**
+ * @typedef {object} LookupConfig Configuration used to lookup a file.
+ * @property {string | null} [path] The base path to search within.
+ * @property {string | null} [name] The name of the file without an extension.
+ * @property {string | Array<string> | ExtensionMap | null} [extensions] The extensions the file being looked up may have.
+ * @property {string | null} [cwd] The current working directory that `path + name` is resolved against.
+ * @property {boolean | null} [findUp] Whether file lookup should recurse up the directory structure.
+ */
+
+/**
+ * @typedef {object} Found Information about the found file.
+ * @property {string} path The absolute path of the found file.
+ * @property {string | ExtensionMap} extension The extension that matched from configured extensions.
+ */
+
+/**
+ * Creates an expanded config by joining config & defaults and applying normalization.
+ *
+ * @param {LookupConfig | string} [pathObj] The user-provided configuration for file lookup.
+ * @param {LookupConfig} [defaultObj] Default configuration for file lookup.
+ * @returns Information about the found file or `null` if not found.
+ */
 function expandPath(pathObj, defaultObj) {
   if (isString(pathObj)) {
     pathObj = { path: pathObj };
@@ -14,6 +48,7 @@ function expandPath(pathObj, defaultObj) {
 
   pathObj = defaults(pathObj, defaultObj);
 
+  /** @type string */
   let filePath;
   if (!isString(pathObj.path)) {
     return null;
@@ -33,6 +68,7 @@ function expandPath(pathObj, defaultObj) {
   let extArr = createExtensionArray(pathObj.extensions);
   const extMap = createExtensionMap(pathObj.extensions);
 
+  /** @type string */
   let basedir = isString(pathObj.cwd) ? pathObj.cwd.toString() : ".";
   basedir = path.resolve(expandTilde(basedir));
 
@@ -64,6 +100,12 @@ function expandPath(pathObj, defaultObj) {
   };
 }
 
+/**
+ * Resolves an expanded configuration to a file.
+ *
+ * @param {NonNullable<ReturnType<typeof expandPath>>} expanded The expanded configuration to lookup.
+ * @returns {Found | null} Information about the found file or `null` if it cannot be found.
+ */
 function findWithExpandedPath(expanded) {
   const found = expanded.findUp
     ? findUpFile(expanded.basedir, expanded.path, expanded.extArr)
@@ -74,13 +116,24 @@ function findWithExpandedPath(expanded) {
   }
 
   if (expanded.extMap) {
-    found.extension = pick(expanded.extMap, found.extension);
+    return {
+      ...found,
+      extension: pick(expanded.extMap, found.extension),
+    };
   }
   return found;
 }
 
-function findFile(basedir, relpath, extArr) {
-  const noExtPath = path.resolve(basedir, relpath);
+/**
+ * Non-recursive lookup of file + each extension.
+ *
+ * @param {string} basedir The directory to resolve against.
+ * @param {string} filename The filename (without extension) to resolve.
+ * @param {Array<string>} extArr The extensions to resolve against the filename.
+ * @returns Information about the found file or `null` if not found.
+ */
+function findFile(basedir, filename, extArr) {
+  const noExtPath = path.resolve(basedir, filename);
   for (const ext of extArr) {
     const filepath = noExtPath + ext;
     try {
@@ -94,10 +147,18 @@ function findFile(basedir, relpath, extArr) {
   return null;
 }
 
-function findUpFile(basedir, filepath, extArr) {
+/**
+ * Recursive lookup of file + each extension.
+ *
+ * @param {string} basedir The directory to resolve against.
+ * @param {string} filename The filename (without extension) to resolve.
+ * @param {Array<string>} extArr The extensions to resolve against the filename.
+ * @returns Information about the found file or `null` if not found.
+ */
+function findUpFile(basedir, filename, extArr) {
   let lastdir;
   do {
-    const found = findFile(basedir, filepath, extArr);
+    const found = findFile(basedir, filename, extArr);
     if (found) {
       return found;
     }
@@ -109,24 +170,37 @@ function findUpFile(basedir, filepath, extArr) {
   return null;
 }
 
+/**
+ * Builds an array of extensions to match against. Will always contain `[""]` at minimum.
+ *
+ * @param {string | Array<string> | ExtensionMap | null} [exts] The configured extensions.
+ * @returns {Array<string>} All extensions to lookup.
+ */
 function createExtensionArray(exts) {
   if (isString(exts)) {
     return [exts];
   }
 
   if (Array.isArray(exts)) {
-    exts = exts.filter(isString);
-    return exts.length > 0 ? exts : [""];
+    const filteredExts = exts.filter(isString);
+    return filteredExts.length > 0 ? filteredExts : [""];
   }
 
   if (isObject(exts)) {
-    exts = Object.keys(exts);
-    return exts.length > 0 ? exts : [""];
+    const extKeys = Object.keys(exts);
+    return extKeys.length > 0 ? extKeys : [""];
   }
 
   return [""];
 }
 
+/**
+ * Builds an object mapping of extensions to match against if provided as configuration.
+ * Will always contain `{ "": null }` if an empty object is provided.
+ *
+ * @param {?} exts The configured extensions.
+ * @returns {ExtensionMap | null} Object mapping of extensions if provided or `null` otherwise.
+ */
 function createExtensionMap(exts) {
   if (!isObject(exts)) {
     return null;
@@ -139,6 +213,13 @@ function createExtensionMap(exts) {
   return exts;
 }
 
+/**
+ * Creates a new object with values defaulted if not provided by `userObj`.
+ *
+ * @param {LookupConfig} [userObj] The user-provided object.
+ * @param {LookupConfig} [defaultObj] An object providing default values.
+ * @returns {LookupConfig} The new object with values defaulted.
+ */
 function defaults(userObj, defaultObj) {
   if (!isObject(userObj)) {
     userObj = {};
@@ -155,19 +236,44 @@ function defaults(userObj, defaultObj) {
   return Object.assign({}, defaultObj, filteredObj);
 }
 
+/**
+ * Creates a new object by extracting exactly the matched key, or an empty object if no match.
+ *
+ * @param {{ [s: string]: *; }} map The original object to pick from.
+ * @param {string} match The key to pick.
+ * @returns {{ [s: string]: *; }} The new object with only the matched key, otherwise an empty object if no match.
+ */
 function pick(map, match) {
   const entries = Object.entries(map).filter(([key]) => key === match);
   return Object.fromEntries(entries);
 }
 
+/**
+ * Determines in a value is a non-null, non-array object.
+ *
+ * @param {?} value The value to check.
+ * @returns {value is {}} `true` if the value is a non-null, non-array object.
+ */
 function isObject(value) {
   return value && typeof value === "object" && !Array.isArray(value);
 }
 
+/**
+ * Determines if an object has no keys.
+ *
+ * @param {object} object The object to check.
+ * @returns {boolean} `true` if the object has no keys, otherwise `false`.
+ */
 function isEmpty(object) {
   return !Object.keys(object).length;
 }
 
+/**
+ * Determines if a vale is a string or String object.
+ *
+ * @param {?} value The value to check.
+ * @returns {value is string | String} `true` if the value is a string or String object, otherwise `false`.
+ */
 function isString(value) {
   if (typeof value === "string") {
     return true;
@@ -184,6 +290,12 @@ function isString(value) {
  * Copyright (c) 2015 Jon Schlinkert.
  * Licensed under the MIT license.
  */
+/**
+ * Expands `~` and `~+` at the start of a file path to home or pwd respectively.
+ *
+ * @param {string} filepath The file path to expand.
+ * @returns {string} A file path with `~` and `~+` expanded.
+ */
 function expandTilde(filepath) {
   const home = os.homedir();
 
diff --git a/node.d.ts b/node.d.ts
new file mode 100644
index 0000000..fbfd90f
--- /dev/null
+++ b/node.d.ts
@@ -0,0 +1,19 @@
+declare module "node:process" {
+  function cwd(): string;
+}
+
+declare module "node:os" {
+  function homedir(): string;
+}
+
+declare module "node:path" {
+  function join(p1: string, p2: string): string;
+  function resolve(p1: string, p2?: string): string;
+  function parse(p1: string): { root: string; ext: string };
+  function isAbsolute(p1: string): boolean;
+  function dirname(p1: string): string;
+}
+
+declare module "node:fs" {
+  function statSync(p1: string): asserts p1;
+}
diff --git a/package-lock.json b/package-lock.json
index c5eea55..5b3e116 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,7 +11,8 @@
       "devDependencies": {
         "eslint": "10.11.0",
         "eslint-config-gulp": "7.0.0",
-        "prettier": "3.9.6"
+        "prettier": "3.9.6",
+        "typescript": "6.0.3"
       },
       "engines": {
         "node": ">= 22.15.0"
@@ -994,6 +995,20 @@
         "node": ">= 0.8.0"
       }
     },
+    "node_modules/typescript": {
+      "version": "6.0.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-6.0.3.tgz",
+      "integrity": "sha512-y2TvuxSZPDyQakkFRPZHKFm+KKVqIisdg9/CZwm9ftvKXLP8NRWj38/ODjNbr43SsoXqNuAisEf1GdCxqWcdBw==",
+      "dev": true,
+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
     "node_modules/uri-js": {
       "version": "4.4.1",
       "resolved": "https://registry.npmjs.org/uri-js/-/uri-js-4.4.1.tgz",
diff --git a/package.json b/package.json
index bef4212..e96d617 100644
--- a/package.json
+++ b/package.json
@@ -18,22 +18,28 @@
   },
   "type": "module",
   "main": "./index.js",
+  "types": "./index.d.ts",
   "exports": "./index.js",
   "files": [
     "index.js",
+    "index.d.ts",
+    "index.d.ts.map",
     "LICENSE"
   ],
   "scripts": {
     "format-check": "prettier . --check",
     "format-write": "prettier . --write",
     "lint": "eslint .",
-    "test": "node --test --experimental-test-coverage --test-reporter=spec --test-reporter-destination=stdout --test-reporter=lcov --test-reporter-destination=test/lcov.info \"test/*.js\""
+    "test": "node --test --experimental-test-coverage --test-reporter=spec --test-reporter-destination=stdout --test-reporter=lcov --test-reporter-destination=test/lcov.info \"test/*.js\"",
+    "types-check": "tsc index.js --checkJs --noEmit --types ./node.d.ts",
+    "types-write": "tsc index.js --checkJs --declaration --declarationMap --emitDeclarationOnly --types ./node.d.ts"
   },
   "dependencies": {},
   "devDependencies": {
     "eslint": "10.11.0",
     "eslint-config-gulp": "7.0.0",
-    "prettier": "3.9.6"
+    "prettier": "3.9.6",
+    "typescript": "6.0.3"
   },
   "keywords": [
     "find",