update flatten to use same syntax as flattenDepth.

Delete flattenDepth.

snippets/flatten.md

@@ -1,11 +1,17 @@
 ### flatten
 
-Flattens an array.
+Flattens an array up to the specified depth.
 
-Use a new array, `Array.concat()` and the spread operator (`...`) to cause a shallow denesting of any contained arrays.
+Use recursion, decrementing `depth` by 1 for each level of depth.
+Use `Array.reduce()` and `Array.concat()` to merge elements or arrays.
+Base case, for `depth` equal to `1` stops recursion.
+Omit the second element, `depth` to flatten only to a depth of `1` (single flatten).
 
 ```js
-const flatten = arr => [].concat(...arr);
+const flatten = (arr, depth = 1) =>
+  depth != 1
+    ? arr.reduce((a, v) => a.concat(Array.isArray(v) ? flatten(v, depth - 1) : v), [])
+    : arr.reduce((a, v) => a.concat(v), []);
 ```
 
 ```js