"I would posit here that nothing I could do in best practice (manual front-end testing, unit testing, error handling, etc.) would have caught the offending line."

Running your code through JSLint (or something similar) would have been helpful here. Performing this check before committing code is considered a best practice and it's really easy to setup a pre-commit hook in most version control systems. JSLint has a Node.js mode. It would have said something like "variable used before being defined". http://www.jslint.com/