A user comment pointed me to the "-dev" version. Apparently there is such a file now containing some lines of error checking (not too many):
http://www.diffnow.com/?report=ahx86 for the diff. I compared http://cdn.sencha.io/ext-4.2.0-gpl/ext-debug.js with http://cdn.sencha.io/ext-4.2.0-gpl/ext-dev.js (My Chrome browser crashed a couple of times with these large files at DiffNow and similar online diff tools.)
A download of the latest version 4.2 contains, among many other files:
ext-all.js 1'402 KB
ext-all-debug.js 3'262 KB
ext-all-debug-w-comments.js 6'052 KB
ext-all-dev.js 6'109 KB
So what Sencha needs to do now is to update this page http://www.sencha.com/learn/debugging-ext-js-applications and recommend to include ext-all-dev.js instead of ext-all-debug.js, while keeping ext-all.js for production.
And, does it help?A couple days forward and I upgraded my app (still in development) from ExtJS 4.1 to 4.2. All seemed fine. There were not too many backward-incompatible changes this time. Plus, I have the dev version now which would alert me of problems right?
A screen using 4.1:
<link rel="stylesheet" href="http://cdn.sencha.io/ext-4.2.0-gpl/resources/css/ext-all-gray.css"/>
The search field on the toolbar is missing. Just gone. Nothing in the console. The grid search extension is from here but that is irrelevant.
Another week passed by, and another case cost me 15 minutes of searching: I've created a new form panel with just one field, copy-pasted from an official example.
fieldLabel: 'My field',
Loading it in the browser, but the panel remains empty (except for the button bar). No error or warning output of course. What's wrong? Something with the panel dimensions? The panel has no width or height? (ExtJS developers will know...) Trying all kinds of combinations. Turns out the problem is very, very basic. I forgot to specify what kind of field it is (xtype: 'textfield'). The example had it configured in the parent object as 'defaultType', and I did not copy that. So ext was supposed to render an element to the screen, but it was not informed about what kind of element it is. What does it do? Apparently nothing... How about at least logging a warning?