Performance measuring, scroll to top, array flattening

snippets/flatten-array.md

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+### Flatten array
+
+Use recursion.
+Use `reduce()` to get all elements that are not arrays, flatten each element that is an array.
+
+```js
+var flatten = arr =>
+  arr.reduce( (a, v) => a.concat( Array.isArray(v) ? flatten(v) : v ), []);
+```