Fixed error prone checking etc in codebase

snippets/join.md

@@ -10,9 +10,9 @@ Omit the third argument, `end`, to use the same value as `separator` by default.
 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,
     ''
   );
 ```