Added a few type checking snippets from lodash

snippets/isWeakMap.md

@@ -0,0 +1,13 @@
+### isWeakMap
+
+Checks if value is classified as a WeakMap object.
+
+Use the `instanceof`operator to check if the provided value is a `WeakMap` object.
+
+```js
+const isWeakMap = val => val instanceof WeakMap;
+```
+
+```js
+isWeakMap(new WeakMap()); // true
+```