diff --git a/index.d.ts b/index.d.ts
index 7412fcf..7bf9cde 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,5 +1,21 @@
-declare function deepmerge<T>(x: Partial<T>, y: Partial<T>, options?: deepmerge.Options): T;
-declare function deepmerge<T1, T2>(x: Partial<T1>, y: Partial<T2>, options?: deepmerge.Options): T1 & T2;
+type ValueOfKey<T, K> = K extends keyof T ? T[K] : never;
+// Is object AND NOT ('never' or 'array')
+type IsObject<T> = [T] extends [never] ? false : T extends unknown[] ? false : [T] extends [object] ? true : false;
+type ShouldMergeProps<T1, T2> = IsObject<T1> extends false ? false : IsObject<T2> extends false ? false : true;
+type DeepMerge<T1, T2> = ShouldMergeProps<T1, T2> extends true
+    ? {
+          [K in keyof T1 | keyof T2]: DeepMerge<ValueOfKey<T1, K>, ValueOfKey<T2, K>>;
+      }
+    : T1 | T2;
+
+/**
+* ## JS Object Deep Merge
+* look more info in https://github.com/TehShrike/deepmerge
+* @param x the first obj
+* @param y the second obj
+* @param options deepmerge option
+*/
+declare function deepmerge<T1 extends object, T2 extends object>(x: T1, y: T2, options?: deepmerge.Options): DeepMerge<T1, T2>
 
 declare namespace deepmerge {
 	export interface Options {
diff --git a/package-lock.json b/package-lock.json
index c112ccb..b809bad 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -328,7 +328,7 @@
         "esprima": "^2.7.3",
         "estraverse": "^4.2.0",
         "marked": "^0.7.0",
-        "v8-argv": "github:jkroso/v8-argv#1.1.1"
+        "v8-argv": "github:jkroso/v8-argv#284f84379e292eb956a5e7b66fb953ec4974385e"
       }
     },
     "levn": {
@@ -644,9 +644,9 @@
       }
     },
     "typescript": {
-      "version": "2.2.2",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-2.2.2.tgz",
-      "integrity": "sha1-YGAiUIR5tV/6NotY/uljoD39eww=",
+      "version": "3.9.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-3.9.3.tgz",
+      "integrity": "sha512-D/wqnB2xzNFIcoBG9FG8cXRDjiqSTbG2wd8DMZeQyJlP1vfTkIxH4GKveWaEBYySKIg+USu+E+EDIR47SqnaMQ==",
       "dev": true
     },
     "uglify-js": {
diff --git a/package.json b/package.json
index 99ed0d6..506e00a 100644
--- a/package.json
+++ b/package.json
@@ -35,7 +35,7 @@
     "rollup-plugin-node-resolve": "^5.2.0",
     "tape": "^4.11.0",
     "ts-node": "7.0.1",
-    "typescript": "=2.2.2",
+    "typescript": "^3.9.3",
     "uglify-js": "^3.6.1"
   },
   "license": "MIT",
diff --git a/test/typescript.ts b/test/typescript.ts
index 23d2c5c..489cc3d 100644
--- a/test/typescript.ts
+++ b/test/typescript.ts
@@ -63,3 +63,51 @@ merged2 = merge(x, z, options2);
 merged3 = merge.all<{wat: number}>([x, y, z], options1);
 
 const merged4 = merge(x, y, options3);
+
+const t1 = {
+	key1: 'key1-val',
+	key2: {
+			key2key1: [false, 'true']
+	},
+	key3: {
+			key3key1: {
+				key3key1key1: 'key3key1key1-val',
+				key3key1key2: true,
+				key3key1key3: ['key3key1key3-val']
+			},
+			key3key2: ['key3key2-val']
+	}
+};
+
+const t2 = {
+	key1: ['key1-val'],
+	key2: {
+			key2key1: {
+					key2key1key1: () => 'key2key1key1-val',
+					key2key1key2: true,
+					key2key1key3: ['key2key1key3-val']
+			}
+	},
+	key3: {
+			key3key1: () => false,
+			key3key2: ['key3key2-val', true]
+	}
+};
+
+const merged5: {
+	key1: string | string[];
+	key2: {
+			key2key1: (string | boolean)[] | {
+					key2key1key1: () => string;
+					key2key1key2: boolean;
+					key2key1key3: string[];
+			};
+	};
+	key3: {
+			key3key1: {
+					key3key1key1: string;key3key1key2: boolean;
+					key3key1key3: string[];
+			};
+			key3key2: (string | boolean)[] | string[];
+	};
+} = merge(t1, t2);