buffer: fix comments in bidirectionalIndexOf

PR-URL: https://github.com/nodejs/node/pull/10162
Fixes: https://github.com/nodejs/node/issues/9801
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

lib/buffer.js

@@ -580,9 +580,10 @@ function bidirectionalIndexOf(buffer, val, byteOffset, encoding, dir) {
   } else if (byteOffset < -0x80000000) {
     byteOffset = -0x80000000;
   }
-  byteOffset = +byteOffset;  // Coerce to Number.
-  if (isNaN(byteOffset)) {
-    // If the offset is undefined, null, NaN, "foo", etc, search whole buffer.
+  // Coerce to Number. Values like null and [] become 0.
+  byteOffset = +byteOffset;
+  // If the offset is undefined, "foo", {}, coerces to NaN, search whole buffer.
+  if (Number.isNaN(byteOffset)) {
     byteOffset = dir ? 0 : (buffer.length - 1);
   }
   dir = !!dir;  // Cast to bool.