diff --git a/lib/lru.js b/lib/lru.js
index 535ee04..454e2b1 100644
--- a/lib/lru.js
+++ b/lib/lru.js
@@ -13,6 +13,9 @@ const assert = require('bsert');
  */
 
 class LRU {
+  /** @type {Map} */
+  map;
+
   /**
    * Create an LRU cache.
    * @constructor
@@ -27,6 +30,7 @@ class LRU {
     assert(!getSize || typeof getSize === 'function', 'Bad size callback.');
     assert(!CustomMap || typeof CustomMap === 'function');
 
+    // @ts-ignore
     this.map = CustomMap ? new CustomMap() : new Map();
     this.size = 0;
     this.items = 0;
@@ -202,20 +206,10 @@ class LRU {
     return true;
   }
 
-  /**
-   * Prepend an item to the linked list (sets new head).
-   * @private
-   * @param {LRUItem}
-   */
-
-  _prependList(item) {
-    this._insertList(null, item);
-  }
-
   /**
    * Append an item to the linked list (sets new tail).
    * @private
-   * @param {LRUItem}
+   * @param {LRUItem} item
    */
 
   _appendList(item) {
@@ -259,7 +253,7 @@ class LRU {
   /**
    * Remove item from the linked list.
    * @private
-   * @param {LRUItem}
+   * @param {LRUItem} item
    */
 
   _removeList(item) {
@@ -420,8 +414,7 @@ class LRUItem {
   /**
    * Create an LRU item.
    * @constructor
-   * @private
-   * @param {String} key
+   * @param {String|Number} key
    * @param {Object} value
    */
 
diff --git a/package-lock.json b/package-lock.json
index 87948de..122dd77 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,7 +12,8 @@
         "bsert": "~0.0.12"
       },
       "devDependencies": {
-        "bmocha": "^2.1.8"
+        "bmocha": "^2.1.8",
+        "bts-type-deps": "^0.0.3"
       },
       "engines": {
         "node": ">=8.0.0"
@@ -38,6 +39,12 @@
       "engines": {
         "node": ">=8.0.0"
       }
+    },
+    "node_modules/bts-type-deps": {
+      "version": "0.0.3",
+      "resolved": "https://registry.npmjs.org/bts-type-deps/-/bts-type-deps-0.0.3.tgz",
+      "integrity": "sha512-OQHGWhX5amae6Vj6ShlGaQu0sNCICgJ5YspNZPRzfR5RobrD+wjm5vkZK/J3EH5b/ymxqSWo9VkiFNpCxjaG2Q==",
+      "dev": true
     }
   }
 }
diff --git a/package.json b/package.json
index 459f7da..60923d3 100644
--- a/package.json
+++ b/package.json
@@ -16,13 +16,15 @@
   "main": "./lib/blru.js",
   "scripts": {
     "lint": "eslint lib/ test/",
+    "lint-types": "tsc -p .",
     "test": "bmocha --reporter spec test/*-test.js"
   },
   "dependencies": {
     "bsert": "~0.0.12"
   },
   "devDependencies": {
-    "bmocha": "^2.1.8"
+    "bmocha": "^2.1.8",
+    "bts-type-deps": "^0.0.3"
   },
   "engines": {
     "node": ">=8.0.0"
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..da7a94b
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,31 @@
+{
+  "include": [
+    "lib/**/*.js"
+  ],
+  "compilerOptions": {
+    "rootDir": ".",
+    "target": "ES2020",
+    "lib": [
+      "ES2020"
+    ],
+
+    "noEmit": true,
+
+    "allowJs": true,
+    "checkJs": true,
+    "maxNodeModuleJsDepth": 10,
+
+    "module": "commonjs",
+    "moduleResolution": "node",
+    "resolveJsonModule": true,
+
+    "stripInternal": true,
+    "noImplicitThis": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": false,
+
+    "typeRoots": [
+      "node_modules/bts-type-deps/types"
+    ]
+  }
+}