Fixed error prone checking etc in codebase

test/flatten/flatten.js

@@ -1,5 +1,5 @@
 const flatten = (arr, depth = 1) =>
-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), []);
 module.exports = flatten;