diff --git a/lib/lru.d.ts b/lib/lru.d.ts
new file mode 100644
index 0000000..204d0ee
--- /dev/null
+++ b/lib/lru.d.ts
@@ -0,0 +1,18 @@
+declare class LRU<T = any> {
+    private first;
+    private items;
+    private last;
+    private max;
+    private size;
+    private ttl;
+    constructor(max?: number, ttl?: number);
+    has(key: string): boolean;
+    clear(): this;
+    delete(key: string): this;
+    evict(): this;
+    get(key: string): T;
+    keys(): string[];
+    set(key: string, value: T, bypass?: boolean): this;
+}
+export default function factory(max?: number, ttl?: number): LRU<any>;
+export {};
diff --git a/lib/tiny-lru.cjs.js b/lib/tiny-lru.cjs.js
index 712da54..50be73a 100644
--- a/lib/tiny-lru.cjs.js
+++ b/lib/tiny-lru.cjs.js
@@ -1,151 +1,120 @@
 'use strict';
 
-class LRU {
-		constructor (max = 0, ttl = 0) {
-			this.first = null;
-			this.items = {};
-			this.last = null;
-			this.max = max;
-			this.size = 0;
-			this.ttl = ttl;
-		}
-
-		has (key) {
-			return key in this.items;
-		}
-
-		clear () {
-			this.first = null;
-			this.items = {};
-			this.last = null;
-			this.size = 0;
-
-			return this;
-		}
-
-		delete (key) {
-			if (this.has(key)) {
-				const item = this.items[key];
-
-				delete this.items[key];
-				this.size--;
-
-				if (item.prev !== null) {
-					item.prev.next = item.next;
-				}
-
-				if (item.next !== null) {
-					item.next.prev = item.prev;
-				}
-
-				if (this.first === item) {
-					this.first = item.next;
-				}
-
-				if (this.last === item) {
-					this.last = item.prev;
-				}
-			}
-
-			return this;
-		}
-
-		evict () {
-			const item = this.first;
-
-			delete this.items[item.key];
-			this.first = item.next;
-			this.first.prev = null;
-			this.size--;
-
-			return this;
-		}
-
-		get (key) {
-			let result;
-
-			if (this.has(key)) {
-				const item = this.items[key];
-
-				if (this.ttl > 0 && item.expiry <= new Date().getTime()) {
-					this.delete(key);
-				} else {
-					result = item.value;
-					this.set(key, result, true);
-				}
-			}
-
-			return result;
-		}
-
-		keys () {
-			return Object.keys(this.items);
-		}
-
-		set (key, value, bypass = false) {
-			let item;
-
-			if (bypass || this.has(key)) {
-				item = this.items[key];
-				item.value = value;
-
-				if (this.last !== item) {
-					const last = this.last,
-						next = item.next,
-						prev = item.prev;
-
-					if (this.first === item) {
-						this.first = item.next;
-					}
-
-					item.next = null;
-					item.prev = this.last;
-					last.next = item;
-
-					if (prev !== null) {
-						prev.next = next;
-					}
-
-					if (next !== null) {
-						next.prev = prev;
-					}
-				}
-			} else {
-				if (this.max > 0 && this.size === this.max) {
-					this.evict();
-				}
-
-				item = this.items[key] = {
-					expiry: this.ttl > 0 ? new Date().getTime() + this.ttl : this.ttl,
-					key: key,
-					prev: this.last,
-					next: null,
-					value
-				};
-
-				if (++this.size === 1) {
-					this.first = item;
-				} else {
-					this.last.next = item;
-				}
-			}
-
-			this.last = item;
-
-			return this;
-		}
-	}
-
-	function factory (max = 1000, ttl = 0) {
-		if (isNaN(max) || max < 0) {
-			throw new TypeError("Invalid max value");
-		}
-
-		if (isNaN(ttl) || ttl < 0) {
-			throw new TypeError("Invalid ttl value");
-		}
-
-		return new LRU(max, ttl);
-	}
+class LRU {
+    constructor(max = 0, ttl = 0) {
+        this.first = null;
+        this.items = {};
+        this.last = null;
+        this.max = max;
+        this.size = 0;
+        this.ttl = ttl;
+    }
+    has(key) {
+        return key in this.items;
+    }
+    clear() {
+        this.first = null;
+        this.items = {};
+        this.last = null;
+        this.size = 0;
+        return this;
+    }
+    delete(key) {
+        if (this.has(key)) {
+            const item = this.items[key];
+            delete this.items[key];
+            this.size--;
+            if (item.prev !== null) {
+                item.prev.next = item.next;
+            }
+            if (item.next !== null) {
+                item.next.prev = item.prev;
+            }
+            if (this.first === item) {
+                this.first = item.next;
+            }
+            if (this.last === item) {
+                this.last = item.prev;
+            }
+        }
+        return this;
+    }
+    evict() {
+        const item = this.first;
+        delete this.items[item.key];
+        this.first = item.next;
+        this.first.prev = null;
+        this.size--;
+        return this;
+    }
+    get(key) {
+        let result;
+        if (this.has(key)) {
+            const item = this.items[key];
+            if (this.ttl > 0 && item.expiry <= new Date().getTime()) {
+                this.delete(key);
+            }
+            else {
+                result = item.value;
+                this.set(key, result, true);
+            }
+        }
+        return result;
+    }
+    keys() {
+        return Object.keys(this.items);
+    }
+    set(key, value, bypass = false) {
+        let item;
+        if (bypass || this.has(key)) {
+            item = this.items[key];
+            item.value = value;
+            if (this.last !== item) {
+                const last = this.last, next = item.next, prev = item.prev;
+                if (this.first === item) {
+                    this.first = item.next;
+                }
+                item.next = null;
+                item.prev = this.last;
+                last.next = item;
+                if (prev !== null) {
+                    prev.next = next;
+                }
+                if (next !== null) {
+                    next.prev = prev;
+                }
+            }
+        }
+        else {
+            if (this.max > 0 && this.size === this.max) {
+                this.evict();
+            }
+            item = this.items[key] = {
+                expiry: this.ttl > 0 ? new Date().getTime() + this.ttl : this.ttl,
+                key: key,
+                prev: this.last,
+                next: null,
+                value
+            };
+            if (++this.size === 1) {
+                this.first = item;
+            }
+            else {
+                this.last.next = item;
+            }
+        }
+        this.last = item;
+        return this;
+    }
+}
+function factory(max = 1000, ttl = 0) {
+    if (isNaN(max) || max < 0) {
+        throw new TypeError("Invalid max value");
+    }
+    if (isNaN(ttl) || ttl < 0) {
+        throw new TypeError("Invalid ttl value");
+    }
+    return new LRU(max, ttl);
+}
 
 module.exports = factory;
diff --git a/lib/tiny-lru.esm.js b/lib/tiny-lru.esm.js
index c14a83f..b37987e 100644
--- a/lib/tiny-lru.esm.js
+++ b/lib/tiny-lru.esm.js
@@ -1,149 +1,118 @@
-class LRU {
-		constructor (max = 0, ttl = 0) {
-			this.first = null;
-			this.items = {};
-			this.last = null;
-			this.max = max;
-			this.size = 0;
-			this.ttl = ttl;
-		}
-
-		has (key) {
-			return key in this.items;
-		}
-
-		clear () {
-			this.first = null;
-			this.items = {};
-			this.last = null;
-			this.size = 0;
-
-			return this;
-		}
-
-		delete (key) {
-			if (this.has(key)) {
-				const item = this.items[key];
-
-				delete this.items[key];
-				this.size--;
-
-				if (item.prev !== null) {
-					item.prev.next = item.next;
-				}
-
-				if (item.next !== null) {
-					item.next.prev = item.prev;
-				}
-
-				if (this.first === item) {
-					this.first = item.next;
-				}
-
-				if (this.last === item) {
-					this.last = item.prev;
-				}
-			}
-
-			return this;
-		}
-
-		evict () {
-			const item = this.first;
-
-			delete this.items[item.key];
-			this.first = item.next;
-			this.first.prev = null;
-			this.size--;
-
-			return this;
-		}
-
-		get (key) {
-			let result;
-
-			if (this.has(key)) {
-				const item = this.items[key];
-
-				if (this.ttl > 0 && item.expiry <= new Date().getTime()) {
-					this.delete(key);
-				} else {
-					result = item.value;
-					this.set(key, result, true);
-				}
-			}
-
-			return result;
-		}
-
-		keys () {
-			return Object.keys(this.items);
-		}
-
-		set (key, value, bypass = false) {
-			let item;
-
-			if (bypass || this.has(key)) {
-				item = this.items[key];
-				item.value = value;
-
-				if (this.last !== item) {
-					const last = this.last,
-						next = item.next,
-						prev = item.prev;
-
-					if (this.first === item) {
-						this.first = item.next;
-					}
-
-					item.next = null;
-					item.prev = this.last;
-					last.next = item;
-
-					if (prev !== null) {
-						prev.next = next;
-					}
-
-					if (next !== null) {
-						next.prev = prev;
-					}
-				}
-			} else {
-				if (this.max > 0 && this.size === this.max) {
-					this.evict();
-				}
-
-				item = this.items[key] = {
-					expiry: this.ttl > 0 ? new Date().getTime() + this.ttl : this.ttl,
-					key: key,
-					prev: this.last,
-					next: null,
-					value
-				};
-
-				if (++this.size === 1) {
-					this.first = item;
-				} else {
-					this.last.next = item;
-				}
-			}
-
-			this.last = item;
-
-			return this;
-		}
-	}
-
-	function factory (max = 1000, ttl = 0) {
-		if (isNaN(max) || max < 0) {
-			throw new TypeError("Invalid max value");
-		}
-
-		if (isNaN(ttl) || ttl < 0) {
-			throw new TypeError("Invalid ttl value");
-		}
-
-		return new LRU(max, ttl);
-	}
+class LRU {
+    constructor(max = 0, ttl = 0) {
+        this.first = null;
+        this.items = {};
+        this.last = null;
+        this.max = max;
+        this.size = 0;
+        this.ttl = ttl;
+    }
+    has(key) {
+        return key in this.items;
+    }
+    clear() {
+        this.first = null;
+        this.items = {};
+        this.last = null;
+        this.size = 0;
+        return this;
+    }
+    delete(key) {
+        if (this.has(key)) {
+            const item = this.items[key];
+            delete this.items[key];
+            this.size--;
+            if (item.prev !== null) {
+                item.prev.next = item.next;
+            }
+            if (item.next !== null) {
+                item.next.prev = item.prev;
+            }
+            if (this.first === item) {
+                this.first = item.next;
+            }
+            if (this.last === item) {
+                this.last = item.prev;
+            }
+        }
+        return this;
+    }
+    evict() {
+        const item = this.first;
+        delete this.items[item.key];
+        this.first = item.next;
+        this.first.prev = null;
+        this.size--;
+        return this;
+    }
+    get(key) {
+        let result;
+        if (this.has(key)) {
+            const item = this.items[key];
+            if (this.ttl > 0 && item.expiry <= new Date().getTime()) {
+                this.delete(key);
+            }
+            else {
+                result = item.value;
+                this.set(key, result, true);
+            }
+        }
+        return result;
+    }
+    keys() {
+        return Object.keys(this.items);
+    }
+    set(key, value, bypass = false) {
+        let item;
+        if (bypass || this.has(key)) {
+            item = this.items[key];
+            item.value = value;
+            if (this.last !== item) {
+                const last = this.last, next = item.next, prev = item.prev;
+                if (this.first === item) {
+                    this.first = item.next;
+                }
+                item.next = null;
+                item.prev = this.last;
+                last.next = item;
+                if (prev !== null) {
+                    prev.next = next;
+                }
+                if (next !== null) {
+                    next.prev = prev;
+                }
+            }
+        }
+        else {
+            if (this.max > 0 && this.size === this.max) {
+                this.evict();
+            }
+            item = this.items[key] = {
+                expiry: this.ttl > 0 ? new Date().getTime() + this.ttl : this.ttl,
+                key: key,
+                prev: this.last,
+                next: null,
+                value
+            };
+            if (++this.size === 1) {
+                this.first = item;
+            }
+            else {
+                this.last.next = item;
+            }
+        }
+        this.last = item;
+        return this;
+    }
+}
+function factory(max = 1000, ttl = 0) {
+    if (isNaN(max) || max < 0) {
+        throw new TypeError("Invalid max value");
+    }
+    if (isNaN(ttl) || ttl < 0) {
+        throw new TypeError("Invalid ttl value");
+    }
+    return new LRU(max, ttl);
+}
 
 export default factory;
diff --git a/package-lock.json b/package-lock.json
index df4fee9..bbd48cd 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -14,18 +14,18 @@
       }
     },
     "@babel/core": {
-      "version": "7.6.2",
-      "resolved": "https://registry.npmjs.org/@babel/core/-/core-7.6.2.tgz",
-      "integrity": "sha512-l8zto/fuoZIbncm+01p8zPSDZu/VuuJhAfA7d/AbzM09WR7iVhavvfNDYCNpo1VvLk6E6xgAoP9P+/EMJHuRkQ==",
+      "version": "7.6.4",
+      "resolved": "https://registry.npmjs.org/@babel/core/-/core-7.6.4.tgz",
+      "integrity": "sha512-Rm0HGw101GY8FTzpWSyRbki/jzq+/PkNQJ+nSulrdY6gFGOsNseCqD6KHRYe2E+EdzuBdr2pxCp6s4Uk6eJ+XQ==",
       "dev": true,
       "requires": {
         "@babel/code-frame": "^7.5.5",
-        "@babel/generator": "^7.6.2",
+        "@babel/generator": "^7.6.4",
         "@babel/helpers": "^7.6.2",
-        "@babel/parser": "^7.6.2",
+        "@babel/parser": "^7.6.4",
         "@babel/template": "^7.6.0",
-        "@babel/traverse": "^7.6.2",
-        "@babel/types": "^7.6.0",
+        "@babel/traverse": "^7.6.3",
+        "@babel/types": "^7.6.3",
         "convert-source-map": "^1.1.0",
         "debug": "^4.1.0",
         "json5": "^2.1.0",
@@ -33,6 +33,54 @@
         "resolve": "^1.3.2",
         "semver": "^5.4.1",
         "source-map": "^0.5.0"
+      },
+      "dependencies": {
+        "@babel/generator": {
+          "version": "7.6.4",
+          "resolved": "https://registry.npmjs.org/@babel/generator/-/generator-7.6.4.tgz",
+          "integrity": "sha512-jsBuXkFoZxk0yWLyGI9llT9oiQ2FeTASmRFE32U+aaDTfoE92t78eroO7PTpU/OrYq38hlcDM6vbfLDaOLy+7w==",
+          "dev": true,
+          "requires": {
+            "@babel/types": "^7.6.3",
+            "jsesc": "^2.5.1",
+            "lodash": "^4.17.13",
+            "source-map": "^0.5.0"
+          }
+        },
+        "@babel/parser": {
+          "version": "7.6.4",
+          "resolved": "https://registry.npmjs.org/@babel/parser/-/parser-7.6.4.tgz",
+          "integrity": "sha512-D8RHPW5qd0Vbyo3qb+YjO5nvUVRTXFLQ/FsDxJU2Nqz4uB5EnUN0ZQSEYpvTIbRuttig1XbHWU5oMeQwQSAA+A==",
+          "dev": true
+        },
+        "@babel/traverse": {
+          "version": "7.6.3",
+          "resolved": "https://registry.npmjs.org/@babel/traverse/-/traverse-7.6.3.tgz",
+          "integrity": "sha512-unn7P4LGsijIxaAJo/wpoU11zN+2IaClkQAxcJWBNCMS6cmVh802IyLHNkAjQ0iYnRS3nnxk5O3fuXW28IMxTw==",
+          "dev": true,
+          "requires": {
+            "@babel/code-frame": "^7.5.5",
+            "@babel/generator": "^7.6.3",
+            "@babel/helper-function-name": "^7.1.0",
+            "@babel/helper-split-export-declaration": "^7.4.4",
+            "@babel/parser": "^7.6.3",
+            "@babel/types": "^7.6.3",
+            "debug": "^4.1.0",
+            "globals": "^11.1.0",
+            "lodash": "^4.17.13"
+          }
+        },
+        "@babel/types": {
+          "version": "7.6.3",
+          "resolved": "https://registry.npmjs.org/@babel/types/-/types-7.6.3.tgz",
+          "integrity": "sha512-CqbcpTxMcpuQTMhjI37ZHVgjBkysg5icREQIEZ0eG1yCNwg3oy+5AaLiOKmjsCj6nqOsa6Hf0ObjRVwokb7srA==",
+          "dev": true,
+          "requires": {
+            "esutils": "^2.0.2",
+            "lodash": "^4.17.13",
+            "to-fast-properties": "^2.0.0"
+          }
+        }
       }
     },
     "@babel/generator": {
@@ -120,9 +168,9 @@
       }
     },
     "@babel/runtime": {
-      "version": "7.6.2",
-      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.6.2.tgz",
-      "integrity": "sha512-EXxN64agfUqqIGeEjI5dL5z0Sw0ZwWo1mLTi4mQowCZ42O59b7DRpZAnTC6OqdF28wMBMFKNb/4uFGrVaigSpg==",
+      "version": "7.6.3",
+      "resolved": "https://registry.npmjs.org/@babel/runtime/-/runtime-7.6.3.tgz",
+      "integrity": "sha512-kq6anf9JGjW8Nt5rYfEuGRaEAaH1mkv3Bbu6rYvLOpPh/RusSJXuKPEAoZ7L7gybZkchE8+NV5g9vKF4AGAtsA==",
       "dev": true,
       "requires": {
         "regenerator-runtime": "^0.13.2"
@@ -615,28 +663,28 @@
       }
     },
     "@nodelib/fs.scandir": {
-      "version": "2.1.2",
-      "resolved": "https://registry.npmjs.org/@nodelib/fs.scandir/-/fs.scandir-2.1.2.tgz",
-      "integrity": "sha512-wrIBsjA5pl13f0RN4Zx4FNWmU71lv03meGKnqRUoCyan17s4V3WL92f3w3AIuWbNnpcrQyFBU5qMavJoB8d27w==",
+      "version": "2.1.3",
+      "resolved": "https://registry.npmjs.org/@nodelib/fs.scandir/-/fs.scandir-2.1.3.tgz",
+      "integrity": "sha512-eGmwYQn3gxo4r7jdQnkrrN6bY478C3P+a/y72IJukF8LjB6ZHeB3c+Ehacj3sYeSmUXGlnA67/PmbM9CVwL7Dw==",
       "dev": true,
       "requires": {
-        "@nodelib/fs.stat": "2.0.2",
+        "@nodelib/fs.stat": "2.0.3",
         "run-parallel": "^1.1.9"
       }
     },
     "@nodelib/fs.stat": {
-      "version": "2.0.2",
-      "resolved": "https://registry.npmjs.org/@nodelib/fs.stat/-/fs.stat-2.0.2.tgz",
-      "integrity": "sha512-z8+wGWV2dgUhLqrtRYa03yDx4HWMvXKi1z8g3m2JyxAx8F7xk74asqPk5LAETjqDSGLFML/6CDl0+yFunSYicw==",
+      "version": "2.0.3",
+      "resolved": "https://registry.npmjs.org/@nodelib/fs.stat/-/fs.stat-2.0.3.tgz",
+      "integrity": "sha512-bQBFruR2TAwoevBEd/NWMoAAtNGzTRgdrqnYCc7dhzfoNvqPzLyqlEQnzZ3kVnNrSp25iyxE00/3h2fqGAGArA==",
       "dev": true
     },
     "@nodelib/fs.walk": {
-      "version": "1.2.3",
-      "resolved": "https://registry.npmjs.org/@nodelib/fs.walk/-/fs.walk-1.2.3.tgz",
-      "integrity": "sha512-l6t8xEhfK9Sa4YO5mIRdau7XSOADfmh3jCr0evNHdY+HNkW6xuQhgMH7D73VV6WpZOagrW0UludvMTiifiwTfA==",
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+      "resolved": "https://registry.npmjs.org/@nodelib/fs.walk/-/fs.walk-1.2.4.tgz",
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@@ -7066,40 +7112,63 @@
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@@ -7208,12 +7277,6 @@
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diff --git a/package.json b/package.json
index ba49842..0c081e9 100644
--- a/package.json
+++ b/package.json
@@ -16,11 +16,11 @@
     "*.d.ts"
   ],
   "license": "BSD-3-Clause",
-  "source": "src/lru.js",
+  "source": "src/lru.ts",
   "browser": "lib/tiny-lru.js",
   "main": "lib/tiny-lru.cjs.js",
   "module": "lib/tiny-lru.esm.js",
-  "types": "tiny-lru.d.ts",
+  "types": "lib/tiny-lru.d.ts",
   "engines": {
     "node": ">=6"
   },
@@ -33,7 +33,7 @@
   },
   "dependencies": {},
   "devDependencies": {
-    "tslib-cli": "^5.0.18",
+    "tslib-cli": "^5.2.2",
     "nodeunit": "^0.11.3",
     "precise": "^1.1.0"
   },
diff --git a/src/lru.js b/src/lru.ts
similarity index 90%
rename from src/lru.js
rename to src/lru.ts
index 62d8367..5616f39 100644
--- a/src/lru.js
+++ b/src/lru.ts
@@ -1,4 +1,12 @@
-	class LRU {
+	class LRU<T = any> {
+
+		private first;
+		private items;
+		private last;
+		private max;
+		private size;
+		private ttl;
+
 		constructor (max = 0, ttl = 0) {
 			this.first = null;
 			this.items = {};
@@ -8,7 +16,7 @@
 			this.ttl = ttl;
 		}
 
-		has (key) {
+		has (key: string) {
 			return key in this.items;
 		}
 
@@ -21,7 +29,7 @@
 			return this;
 		}
 
-		delete (key) {
+		delete (key: string) {
 			if (this.has(key)) {
 				const item = this.items[key];
 
@@ -59,7 +67,7 @@
 			return this;
 		}
 
-		get (key) {
+		get (key: string): T {
 			let result;
 
 			if (this.has(key)) {
@@ -80,7 +88,7 @@
 			return Object.keys(this.items);
 		}
 
-		set (key, value, bypass = false) {
+		set (key: string, value: T, bypass = false) {
 			let item;
 
 			if (bypass || this.has(key)) {
diff --git a/tiny-lru.d.ts b/tiny-lru.d.ts
deleted file mode 100644
index f4b2f34..0000000
--- a/tiny-lru.d.ts
+++ /dev/null
@@ -1,14 +0,0 @@
-declare module "tiny-lru" {
-  export class Lru<T = any> {
-    constructor(max?: number, ttl?: number);
-
-    public has(key: string): boolean;
-    public get(key: string): T;
-    public set(key: string, value: T, bypass?: boolean): this;
-    public clear(): this;
-    public delete(key: string): this;
-    public evict(): this;
-    public keys(): string[];
-  }
-  export default Lru;
-}
diff --git a/tsconfig.json b/tsconfig.json
index f38ddc4..2b5a596 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,13 +1,14 @@
 {
 	"exclude": ["node_modules", "lib"],
 	"compilerOptions": {
+		"declaration": true,
+		"declarationDir": "lib",
 		"lib": ["dom", "esnext"],
 		"module": "esnext",
 		"moduleResolution": "node",
 		"outDir": "lib",
 		"strict": false,
 		"target": "esnext",
-		"allowJs": true,
 		"sourceMap": true
 	}
 }
\ No newline at end of file