diff --git a/Makefile b/Makefile
index c6f1829..c441dc6 100644
--- a/Makefile
+++ b/Makefile
@@ -7,6 +7,7 @@ TESTS_IN = test/*.test.js
 
 # Tools
 standard = $(BIN)/standard
+tsc = $(BIN)/tsc
 snazzy = $(BIN)/snazzy
 coveralls = $(BIN)/coveralls
 istanbul = $(BIN)/istanbul
@@ -19,6 +20,10 @@ uglifyjs = $(BIN)/uglifyjs
 lint:
 	$(standard) --verbose | $(snazzy)
 
+# Run type checking..
+type-check:
+	$(tsc)
+
 # Save code coverage to coveralls
 coveralls:
 	cat coverage/lcov.info | $(coveralls)
diff --git a/index.d.ts b/index.d.ts
index 9921d49..16f9196 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,4 +1,4 @@
-namespace Receptacle {
+declare namespace Receptacle {
     export interface Options<T> {
         id?: number|string;
         max?: number;
@@ -31,7 +31,7 @@ namespace Receptacle {
     }
 }
 
-class Receptacle<T, X = undefined> {
+declare class Receptacle<T, X = undefined> {
     constructor(options?: Receptacle.Options<T>);
     public id: number|string;
     public max: number;
@@ -40,9 +40,9 @@ class Receptacle<T, X = undefined> {
     public has(key: string): boolean;
     public get(key: string): T|null;
     public meta(key: string): X|undefined;
-    public set(key: string, value: T, options?: Receptacle.SetOptions<X>): Receptacle;
+    public set(key: string, value: T, options?: Receptacle.SetOptions<X>): Receptacle<T, X>;
     public delete(key: string): void;
-    public expire(key: string, ms: number = 0): void;
+    public expire(key: string, ms?: number): void;
     public clear(): void;
     public toJSON(): Receptacle.Export<T, X>;
 }
diff --git a/package-lock.json b/package-lock.json
index 773b512..f8afb9e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -84,6 +84,7 @@
       "resolved": "https://registry.npmjs.org/align-text/-/align-text-0.1.4.tgz",
       "integrity": "sha1-DNkKVhCT810KmSVsIrcGlDP60Rc=",
       "dev": true,
+      "optional": true,
       "requires": {
         "kind-of": "^3.0.2",
         "longest": "^1.0.1",
@@ -2368,6 +2369,7 @@
       "resolved": "https://registry.npmjs.org/kind-of/-/kind-of-3.1.0.tgz",
       "integrity": "sha1-R11pil5J/15T0U4+cyQp3Iv0z0c=",
       "dev": true,
+      "optional": true,
       "requires": {
         "is-buffer": "^1.0.2"
       }
@@ -2475,7 +2477,8 @@
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/longest/-/longest-1.0.1.tgz",
       "integrity": "sha1-MKCy2jj3N3DoKUoNIuZiXtd9AJc=",
-      "dev": true
+      "dev": true,
+      "optional": true
     },
     "loose-envify": {
       "version": "1.3.1",
@@ -3176,7 +3179,8 @@
       "version": "1.6.1",
       "resolved": "https://registry.npmjs.org/repeat-string/-/repeat-string-1.6.1.tgz",
       "integrity": "sha1-jcrkcOHIirwtYA//Sndihtp15jc=",
-      "dev": true
+      "dev": true,
+      "optional": true
     },
     "request": {
       "version": "2.87.0",
@@ -3724,6 +3728,12 @@
       "integrity": "sha1-hnrHTjhkGHsdPUfZlqeOxciDB3c=",
       "dev": true
     },
+    "typescript": {
+      "version": "4.9.5",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.9.5.tgz",
+      "integrity": "sha512-1FXk9E2Hm+QzZQ7z+McJiHL4NW1F2EzMu9Nq9i3zAaGqibafqYwCVU6WyWAuyQRRzOlxou8xZSyXLEN8oKj24g==",
+      "dev": true
+    },
     "ua-parser-js": {
       "version": "0.7.18",
       "resolved": "https://registry.npmjs.org/ua-parser-js/-/ua-parser-js-0.7.18.tgz",
diff --git a/package.json b/package.json
index 137d212..cf25e91 100644
--- a/package.json
+++ b/package.json
@@ -16,6 +16,7 @@
     "mocha": "^5.2.0",
     "snazzy": "^7.1.1",
     "standard": "^11.0.1",
+    "typescript": "^4.9.5",
     "uglifyjs": "^2.4.11"
   },
   "files": [
@@ -42,8 +43,9 @@
     "build": "make build",
     "coveralls": "make coveralls",
     "lint": "make lint",
-    "test": "npm run lint && make test",
-    "test-ci": "npm run lint && make test-ci"
+    "type-check": "make type-check",
+    "test": "npm run lint && npm run type-check && make test",
+    "test-ci": "npm run lint && npm run type-check && make test-ci"
   },
   "typings": "./index.d.ts",
   "standard": {
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..254c7b0
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,3 @@
+{
+  "files": ["index.d.ts"]
+}