Add functions, functionsIn

snippets/functions.md

@@ -0,0 +1,18 @@
+### functions
+
+Returns an array of function property names from own enumerable properties of object.
+
+Use `Object.keys(obj)` to iterate over the object's own properties, `Array.filter()` to keep only those that are functions.
+
+```js
+const functions = obj =>
+  Object.keys(obj).filter(key => typeof obj[key] === 'function');
+```
+
+```js
+function Foo() {
+  this.a = () => 1;
+  this.b = () => 2;
+}
+functions(new Foo); // ['a', 'b']
+```