diff --git a/.gitignore b/.gitignore
index b8f34b2..218840d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 .idea
 coverage/
+node_modules
\ No newline at end of file
diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..0ce63b0
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,44 @@
+// Type definitions for biguint-format
+// Project: biguint-format https://github.com/T-PWK/flake-idgen
+// Definitions by: m-star https://github.com/mstar-kk
+
+/// <reference types="node" />
+
+interface FORMATS {
+  dec: typeof toDecimalString;
+  hex: typeof toHexString;
+  bin: typeof toBinaryString;
+  oct: typeof toOctetString;
+}
+
+interface formatOption {
+  format?: string;
+  groupsize?: number;
+  delimiter?: string;
+  padstr?: string;
+  size?: number;
+  trim?: boolean;
+  prefix?: string;
+}
+
+export default function (
+  buffer: Buffer,
+  base: 'dec' | 'hex' | 'bin' | 'oct',
+  options?: formatOption
+): string;
+
+export function createBuffer(size: number): Buffer;
+
+export function createBufferFrom(object: object): Buffer;
+
+export function toDecimalString(buffer: Buffer, options?: formatOption): string;
+
+export function toBinaryString(buffer: Buffer, options?: formatOption): string;
+
+/*
+ * Converts given input (node Buffer or array of bytes) to hexadecimal string 0xDDDD where D is [0-9a-f].
+ * All leading 0's are stripped out i.e. [0x00, 0x00, 0x00, 0x01] -> '0x1'
+ */
+export function toHexString(buffer: Buffer, options?: formatOption): string;
+
+export function toOctetString(buffer: Buffer, options?: formatOption): string;
diff --git a/package.json b/package.json
index 2e253df..1ab67c9 100644
--- a/package.json
+++ b/package.json
@@ -35,5 +35,8 @@
   ],
   "bugs": {
     "url": "https://github.com/T-PWK/biguint-format/issues"
+  },
+  "devDependencies": {
+    "@types/node": "^16.7.10"
   }
 }