diff --git a/index.d.ts b/index.d.ts
index ee4d39b..731c3d2 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,10 +1,37 @@
-/** Declaration file generated by dts-gen */
-
+/**
+ * truncates a floating point number
+ *
+ * @param {number} num number to truncate
+ * @param {number} len length to truncate to
+ * @returns {number} truncated number
+ */
+export function truncate_decimal(num: number, len: number): number;
+/**
+ * parse json and catch invalid strings
+ *
+ * @param {string} text input
+ * @returns {any} parsed
+ */
+export function try_parse_json(text: string): any;
+/**
+ * copy an object to prevent modification to the original
+ *
+ * @param {object} obj object to copy
+ * @returns {object} copy
+ */
 export function copy_object(obj: any): any;
-
+/**
+ * run a regular expression and callback for every result
+ *
+ * @param {any} regex regular expression
+ * @param {any} data data to run on
+ * @param {any} func function to execute
+ */
 export function run_regex(regex: any, data: any, func: any): void;
-
-export function truncate_decimal(num: any, len: any): any;
-
-export function try_parse_json(text: any): any;
-
+/**
+ * check if an object is either undefined, null or NaN
+ *
+ * @param {any} obj object to check
+ * @returns {boolean} true if nil
+ */
+export function is_nil(obj: any): boolean;
diff --git a/index.js b/index.js
index 3e812b7..a463d36 100644
--- a/index.js
+++ b/index.js
@@ -66,10 +66,22 @@ function run_regex (regex, data, func) {
   }
 }
 
+/**
+ * check if an object is either undefined, null or NaN
+ *
+ * @param {any} obj object to check
+ * @returns {boolean} true if nil
+ */
+function is_nil (obj) {
+  return typeof obj === 'undefined'
+      || obj === null
+      || (typeof obj === 'number' && isNaN (obj));
+}
 
 module.exports = {
   truncate_decimal,
   try_parse_json,
   copy_object,
-  run_regex
+  run_regex,
+  is_nil
 };