diff --git a/package.json b/package.json
index c8cdf8f..9fa1a77 100644
--- a/package.json
+++ b/package.json
@@ -6,6 +6,7 @@
     "name": "Athan Reines",
     "email": "kgryte@gmail.com"
   },
+  "types": "./types/index.d.ts",
   "contributors": [
     {
       "name": "Athan Reines",
diff --git a/types/index.d.ts b/types/index.d.ts
new file mode 100644
index 0000000..79ae1da
--- /dev/null
+++ b/types/index.d.ts
@@ -0,0 +1,27 @@
+/**
+* Accessor function.
+*
+* @param value - input value
+* @returns numerical value
+*/
+type AccessorFunction<T> = (value: T) => number;
+
+/**
+ * Computes the cosine similarity between two arrays.
+ *
+ * @param x - input array
+ * @param y - input array
+ * @param accessor - accessor function for accessing array values
+ * @returns cosine similarity or null
+ */
+declare function similarity(
+  x: number[],
+  y: number[],
+): number | null;
+declare function similarity<T>(
+  x: T[],
+  y: T[],
+  accessor: AccessorFunction<T>
+): number | null;
+
+export default similarity