Use === and !==

+++ b/core/misc/ajax.js
@@ -24,7 +24,7 @@ Drupal.behaviors.AJAX = {
-        if (typeof element_settings.selector == 'undefined') {
+        if (typeof element_settings.selector === 'undefined') {

Wouldn't it make more sense to globally introduce undefined as a undefined variable in our closures?

I.e.:

(function ($, undefined) {
  ...
})(jQuery)

That would make more sense to me, since, AFAIK, that's a JS best practice either way. Separate issue, of course.

+++ b/core/misc/ajax.js
@@ -359,7 +359,7 @@ Drupal.ajax.prototype.beforeSend = function (xmlhttprequest, options) {
-  if (this.progress.type == 'bar') {
+  if (this.progress.type === 'bar') {

@@ -371,7 +371,7 @@ Drupal.ajax.prototype.beforeSend = function (xmlhttprequest, options) {
-  else if (this.progress.type == 'throbber') {
+  else if (this.progress.type === 'throbber') {

I don't really see how a type-agnostic comparison makes the code more robust or more valid in this and other cases.

Drupal.ajax.progress.type defaults to a string and is expected to be a string. If you pass something else (i.e., a different data type), then your code is broken. In general, we don't babysit broken code.

+++ b/core/misc/batch.jsundefined
@@ -10,7 +10,7 @@ Drupal.behaviors.batch = {
-        if (progress == 100) {
+        if (progress === '100') {

do you sure 'progress' is a string, not the number.

Which is weird, PHP should probably return the rightful types?

EDIT: Some other problems may happen, such as a float casted as int or getting out of language granularity returning 100 while it's 99.999999999999999999999999 which could break a very long batch to break before the end, got this kind of errors in some use cases. The right way to fix that is to introduce a boolean "finished" into the server response and test that. While I agree in this particular case sending it as a string would do the trick, it doesn't help for people that tries to understand the code by reading it (it's not explicit). This may need another issue for this specific behavior.

I already suggested to introduce a finished boolean in a another issue. This js code is definitely sloppy.

Oh good to ear that.

Code part looks fine.

Committed to 8.x. Thanks!

Is this backportable or is D7 being left as-is with these logic glitches?

Overlay is borken.

This fixes overlay

Thanks for the fix! Committed and pushed to 8.x. Moving back to 7.x since that's where it was, but I don't really think that this is a good idea for D7, as demonstrated by the fact that we just broke major user-facing functionality in 8.x without knowing it. :P

Great. Kicking back to 8.x and fixed.