In at least Firefox and Chrome, you can use the "debugger;" statement to not only block execution, but actually drop into the interactive debugger.
Indeed, it can be abused as a sort of primitive step-by-step debugging tool.
I remember the days before the "Don't allow this site to show more alerts" checkbox when, if you accidentally left an alert in a loop, your only option was Ctrl-Alt-Delete.
Weird how you had JavaScript support but no process killing. I imagine you must have used DOS. What browser? Arachne didn't have JavaScript support...
Ctrl-Alt-Del is one of the ways to access the task manager in Windows, with which you can kill a process.
You could even instrument your code to implement a full debugger using "prompt".