In theory that could have been caught by a good jslint/syntax checker but those were a few years in the future too. Early, JS development really sucked, its only slightly better today..
I recall a dramatic bug where, in 2012, I rolled out a new version of a web GUI with some JS mixed in it to an intranet audience of IE 7 and Firefox users. It initially looked good, but then reports came in that IE users couldn't use one of the buttons. After much debugging and confusion, it turned out that there was a trailing comma at the end of an array literal (i.e. `[1,2,3,]` instead of `[1,2,3]`). Firefox accepted that, but IE 7 didn't. (I think IE 8 added support for trailing commas.)
Firebug was revolutionary not only for the console but the inspect tool was very needed as layouts and CSS progressed where outputs weren't enough.