Added isX functions

snippets/is-boolean.md

@@ -0,0 +1,9 @@
+### Is boolean
+
+Use `typeof` to check if a value is classified as a boolean primitive.
+
+```js
+const isBoolean = val => typeof val === 'boolean';
+// isBoolean(null) -> false
+// isBoolean(false) -> true
+```