Hey James,
Yes, JS won’t let you use variable names you didn’t declare first (modern engines won’t, at any rate). This goes for variables you’d use in breakpoint conditions, as such conditions are eval’d in the scope of the line the breakpoint is on.
Good to hear you figured it out :-) You could have tried with pre-existing stuff, such as, er, new Date().getMinutes() % 2 == 0 to break only on even minutes (yeah, that’s useless except as a demo).
Best,