diff --git a/.gitignore b/.gitignore
index b0fb09f..73a7d5e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 node_modules
 dist/*
+types/*
diff --git a/package-lock.json b/package-lock.json
index b1cea18..111cca3 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -104,6 +104,16 @@
         }
       }
     },
+    "@rollup/plugin-typescript": {
+      "version": "6.1.0",
+      "resolved": "https://registry.npmjs.org/@rollup/plugin-typescript/-/plugin-typescript-6.1.0.tgz",
+      "integrity": "sha512-hJxaiE6WyNOsK+fZpbFh9CUijZYqPQuAOWO5khaGTUkM8DYNNyA2TDlgamecE+qLOG1G1+CwbWMAx3rbqpp6xQ==",
+      "dev": true,
+      "requires": {
+        "@rollup/pluginutils": "^3.1.0",
+        "resolve": "^1.17.0"
+      }
+    },
     "@rollup/pluginutils": {
       "version": "3.1.0",
       "resolved": "https://registry.npmjs.org/@rollup/pluginutils/-/pluginutils-3.1.0.tgz",
@@ -931,6 +941,12 @@
         "yn": "3.1.1"
       }
     },
+    "tslib": {
+      "version": "2.0.3",
+      "resolved": "https://registry.npmjs.org/tslib/-/tslib-2.0.3.tgz",
+      "integrity": "sha512-uZtkfKblCEQtZKBF6EBXVZeQNl82yqtDQdv+eck8u7tdPxjLu2/lp5/uPW+um2tpuxINHWy3GhiccY7QgEaVHQ==",
+      "dev": true
+    },
     "type-check": {
       "version": "0.3.2",
       "resolved": "https://registry.npmjs.org/type-check/-/type-check-0.3.2.tgz",
@@ -941,9 +957,9 @@
       }
     },
     "typescript": {
-      "version": "2.2.2",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-2.2.2.tgz",
-      "integrity": "sha1-YGAiUIR5tV/6NotY/uljoD39eww=",
+      "version": "4.1.1-rc",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.1.1-rc.tgz",
+      "integrity": "sha512-tgNcFrLIjlaMWEc7bKC0bxLNIt8BIAauY/HLUOQDyTP75HGskETtXOt46x4EKAHRKhWVLMc7yM02puTHa/yhCA==",
       "dev": true
     },
     "v8-argv": {
diff --git a/package.json b/package.json
index c32badc..9d5fe2f 100644
--- a/package.json
+++ b/package.json
@@ -26,15 +26,17 @@
     "readme.md"
   ],
   "scripts": {
-    "build": "rollup -c",
+    "build": "rollup -c && npm run typedef",
     "size": "npm run build && terser --compress --mangle -- ./dist/umd.js | gzip -c | wc -c",
     "test": "npm run build && tape test/*.js && jsmd readme.md && npm run test:typescript",
-    "test:typescript": "tsc --noEmit test/typescript.ts && ts-node test/typescript.ts"
+    "test:typescript": "tsc --noEmit test/typescript.ts && ts-node test/typescript.ts",
+    "typedef": "tsc -p tsconfig.typedef.json"
   },
   "dependencies": {},
   "devDependencies": {
     "@rollup/plugin-commonjs": "^16.0.0",
     "@rollup/plugin-node-resolve": "^10.0.0",
+    "@rollup/plugin-typescript": "^6.1.0",
     "@types/node": "^8.10.54",
     "is-mergeable-object": "1.1.0",
     "is-plain-obj": "^3.0.0",
@@ -43,7 +45,8 @@
     "tape": "^5.0.1",
     "terser": "^5.3.8",
     "ts-node": "^9.0.0",
-    "typescript": "=2.2.2"
+    "tslib": "^2.0.3",
+    "typescript": "^4.1.1-rc"
   },
   "engines": {
     "node": ">=0.10.0"
diff --git a/rollup.config.js b/rollup.config.js
index cd5527c..ec2c616 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -2,13 +2,15 @@
 
 import resolve from '@rollup/plugin-node-resolve'
 import commonjs from '@rollup/plugin-commonjs'
+import typescript from '@rollup/plugin-typescript';
 import pkg from './package.json'
 
 export default {
-	input: `src/index.js`,
+	input: `src/index.ts`,
 	plugins: [
-		commonjs(),
 		resolve(),
+		commonjs(),
+		typescript()
 	],
 	output: [
 		{
diff --git a/src/index.js b/src/index.ts
similarity index 94%
rename from src/index.js
rename to src/index.ts
index 761c002..1db49d1 100644
--- a/src/index.js
+++ b/src/index.ts
@@ -1,4 +1,5 @@
-var isPlainObj = require('is-plain-obj')
+import isPlainObj from 'is-plain-obj';
+
 function defaultIsMergeable(value) {
 	return Array.isArray(value) || isPlainObj(value)
 }
@@ -75,7 +76,7 @@ function mergeObject(target, source, options) {
 	return destination
 }
 
-function deepmerge(target, source, options) {
+export default function deepmerge(target, source, options) {
 	options = Object.assign({
 		arrayMerge: defaultArrayMerge,
 		isMergeable: defaultIsMergeable
@@ -96,7 +97,7 @@ function deepmerge(target, source, options) {
 	}
 }
 
-deepmerge.all = function deepmergeAll(array, options) {
+export function deepmergeAll(array, options) {
 	if (!Array.isArray(array)) {
 		throw new Error('first argument should be an array')
 	}
@@ -105,5 +106,3 @@ deepmerge.all = function deepmergeAll(array, options) {
 		return deepmerge(prev, next, options)
 	}, {})
 }
-
-module.exports = deepmerge
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..8cf05cd
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,17 @@
+{
+	"compilerOptions": {
+		"target": "ES2020",
+		"module": "ES2020",
+		"lib": [
+			"ES2020"
+		],
+		"moduleResolution": "node",
+		"esModuleInterop": true,
+		"strict": true,
+		"skipLibCheck": true,
+		"forceConsistentCasingInFileNames": true
+	},
+	"include": [
+		"src"
+	]
+}
diff --git a/tsconfig.typedef.json b/tsconfig.typedef.json
new file mode 100644
index 0000000..fe082f3
--- /dev/null
+++ b/tsconfig.typedef.json
@@ -0,0 +1,8 @@
+{
+	"extends": "./tsconfig.json",
+	"compilerOptions": {
+		"declaration": true,
+		"declarationDir": "types/",
+		"emitDeclarationOnly": true
+	}
+}
diff --git a/types/index.d.ts b/types/index.d.ts
deleted file mode 100644
index 20f645a..0000000
--- a/types/index.d.ts
+++ /dev/null
@@ -1,16 +0,0 @@
-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;
-
-declare namespace deepmerge {
-	export interface Options {
-		arrayMerge?(target: any[], source: any[], options?: Options): any[];
-		clone?: boolean;
-		customMerge?: (key: string, options?: Options) => ((x: any, y: any) => any) | undefined;
-		isMergeable?(value: object): boolean;
-	}
-
-	export function all (objects: object[], options?: Options): object;
-	export function all<T> (objects: Partial<T>[], options?: Options): T;
-}
-
-export = deepmerge;