Add isAsyncFunction

snippets/isAsyncFunction.md

@@ -0,0 +1,18 @@
+---
+title: isAsyncFunction
+tags: type,function,intermediate
+---
+
+Checks if the given argument is an `async` function.
+
+Use `Object.prototype.toString()` and `Function.call()` and check if the result is `'[object AsyncFunction]'`.
+
+```js
+const isAsyncFunction = val =>
+  Object.prototype.toString.call(val) === '[object AsyncFunction]';
+```
+
+```js
+isAsyncFunction(function() {}); // false
+isAsyncFunction(async function() {}); // true
+```