Added isX functions

snippets/is-function.md

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