"At some point in computer history, somebody (arbitrarily?) created an 80 character line limit for code. ... I’ve been writing JavaScript for three-ish years"
For a couple decades, and not ending until the late 90s, most text terminals and text modes for graphics cards were 80 characters wide[1], and dot matrix printers also had an 80 character line length (plus margins) dating back to 80 characters per line punch cards from 1928[2]. If a line was longer, you had to scroll that individual line. To let your code be read easily anywhere including your own screen, you stuck to that line length.
Which of course begs the question, why were punched cards 80 characters wide? According to a random StackExchange commenter:
"The cards are that size because in 1890, CTR wanted to reuse currency carriers (the dollar was bigger back then) to carry the census data cards. – Al Biglan"[1]
Assuming this is true, and that the size of the card was chosen to match the size of the US dollar, it can perhaps be assumed that 80 columns was chosen as a reasonable compromize between data density and structural integrity. I'm not sure that this makes sense though, since Wikipedia says that IBM's 80-column cards date from 1928. A more authoritative source would be welcome.
Note that typographical measures are usually even smaller, with 66 characters per line being quite common. And those don't use monospaced fonts and are thus even more narrow, and thus easier to scan. Granted, they usually don't include spurious whitespace, but if 40 characters out of your 80 are empty, you've probably got a different problem altogether.
For a couple decades, and not ending until the late 90s, most text terminals and text modes for graphics cards were 80 characters wide[1], and dot matrix printers also had an 80 character line length (plus margins) dating back to 80 characters per line punch cards from 1928[2]. If a line was longer, you had to scroll that individual line. To let your code be read easily anywhere including your own screen, you stuck to that line length.
1. http://en.wikipedia.org/wiki/Text_mode#PC_common_text_modes
2. http://en.wikipedia.org/wiki/Punched_card#IBM_80-column_punc...