diff --git a/index.d.ts b/index.d.ts
index 7412fcf..aa2807c 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,5 +1,50 @@
-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;
+/**
+ * object skin copy
+ * e.g: SkinMerge<{ a: 1, c: 1 }, { a: 2, b: 2 }> ==> { a: 2, b: 2, c: 1 }
+ */
+type SkinMerge<O1, O2> = {
+    [k in (keyof O1 | keyof O2)]: k extends keyof O2 ? O2[k] : (k extends keyof O1 ? O1[k] : never)
+}
+
+/**
+ * get keys from O where the value is object
+ * e.g: ObjKeys<{ a: {}, b: {}, c: '1' }> ==> 'a' | 'b'
+ */
+type ObjKeys<O> = ({
+    [k in keyof O]: O[k] extends object ? k : never;
+})[keyof O];
+
+/**
+ * contrary to ObjKeys, e.g: NotObjKeys<{ a: {}, b: {}, c: '1' }> ==> 'c'
+ */
+type NotObjKeys<O> = Exclude<keyof O, ObjKeys<O>>;
+
+/**
+ * use NotObjKeys<O> to contruct object type from 'O'
+ * e.g: PickNotObj<{ a: 'a', b: 'b', c: {} }> ==> { a: 'a', b: 'b' }
+ */
+type PickNotObj<O> = Pick<O, NotObjKeys<O>>;
+
+/**
+ * Merge Deep Recursively For O1 O2
+ * github gist about MergeDeep: https://gist.github.com/eczn/f037be1fa3650304c5c4b80c3b9e18a7
+ */
+type MergeDeep<O1, O2> = SkinMerge<PickNotObj<O1>, PickNotObj<O2>> & {
+    // k1 is not in ObjKeys<O2>
+    [k1 in Exclude<ObjKeys<O1>, ObjKeys<O2>>]: O1[k1]
+} & {
+    // k2 is ObjKeys<O2>, maybe some ObjKeys<k1> alson in there, so we should find it out
+    [k2 in ObjKeys<O2>]: k2 extends ObjKeys<O1> ? MergeDeep<O1[k2], O2[k2]> : O2[k2]
+}
+
+/**
+ * ## 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): MergeDeep<T1, T2>
 
 declare namespace deepmerge {
 	export interface Options {
diff --git a/package-lock.json b/package-lock.json
index c112ccb..2f64d16 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -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": "3.7.2",
+      "resolved": "http://r.tnpm.oa.com/typescript/download/typescript-3.7.2.tgz",
+      "integrity": "sha1-J+SJuV+lkJRF6f717kjYFpetGPs=",
       "dev": true
     },
     "uglify-js": {
diff --git a/package.json b/package.json
index 99ed0d6..35a10a7 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": "^3.7.2",
     "uglify-js": "^3.6.1"
   },
   "license": "MIT",