Fixed error prone checking etc in codebase

test/join/join.js

@@ -1,9 +1,9 @@
 const join = (arr, separator = ',', end = separator) =>
 arr.reduce(
 (acc, val, i) =>
-i == arr.length - 2
+i === arr.length - 2
 ? acc + val + end
-: i == arr.length - 1 ? acc + val : acc + val + separator,
+: i === arr.length - 1 ? acc + val : acc + val + separator,
 ''
 );
 module.exports = join;