Exercise 12, err parameter

Can you please elaborate on what exactly the “err” parameter in exercise 12 is doing?

I tried breaking the code and seeing how far I could get with running the code without it, and it breaks on line 13:

fs.readFile(“test.txt”, print_lines);

with the error “cannot read properly toString of null.”

wouldn’t the “data” parameter be fulfilled with test.txt.

Thank you for your explanation and I hope my question makes sense.

Can you be more specific about the exact code? Put [code] [/code] around it when you post.