Yes: https://developers.google.com/apps-script/overview
By the way, I started programming in '99, and since I didn't have a computer at the time, I learned JavaScript, since I could run it on IE4 on public machines. So yes, a browser (and, I guess, Notepad) is really all you need!
Besides, since then and between PHP/Python/etc shared hosting and tools like Codeanywhere (which can talk to S/FTP servers), a web browser is enough for building web applications.
(Of course, the web is far from reaching the potential of desktop development, of course. But it has come a long way already)
Neat! I eat my thoughts (It's the kind of thing I miss by using vim for almost everything).
EDIT: But it is a separate app...
"The script editor can be accessed directly at http://script.google.com, or by launching it from one of the Google products which support built-in access to the script editor, such as Google Spreadsheets and Google Sites."
In Spreadsheets, you just need to go to Tools → Scripts → Script Editor.
If you're the kind of person who looks under the hood, you're going to do it regardless of whether you're working on a desktop word processor or a web app, and it is my strong opinion that these kinds of intelligent and curios people are still coming into this world.