I thought I should be able to do this (and according to any Javascript reference I can find, I should):
edge %= lineWidth;
But that just gives me an error about missing the “;”. Glitch, or just not implemented? I’m forced to do this instead:
edge = edge % lineWidth;
The horror!
See, I told you this was a trivial question. 
–Eric
Just out of curiosity I tried this and it does indeed seem like the %= operator isn’t implemented. Anyway I just wanted to say that actually the error message is “Assets/Temp/Temp.js(4) error BCE0043: Unexpected token: =.” which makes a lot more sense. When you get en error always open the console and look at the first error, everything after this is usually just cascading errors and more or less meaningless.
do more:
edge = edge % lineWidth
to get less:
headache = Mathf.Infinity;