Add isNil and isUndefined

snippets/isUndefined.md

@@ -0,0 +1,13 @@
+### isUndefined
+
+Returns `true` if the specified value is `undefined`, `false` otherwise.
+
+Use the strict equality operator to check if the value and of `val` are equal to `undefined`.
+
+```js
+const isUndefined = val => val === undefined;
+```
+
+```js
+isUndefined(undefined); // true
+```