Debugging — reading errors like a pro

You've hit errors all course — that was the point. This lesson turns error-reading from luck into a skill. By the end, a red Output line stops being scary and becomes a precise map of what's wrong.

#Anatomy of an error

Roblox errors follow one shape:

ServerScriptService.MoneySystem:14: attempt to perform arithmetic on a string

Four fields, left to right:

That first field isn't just "which script" — the full path tells you where the script lives, which tells you who can touch it. A bug in a LocalScript is a client problem; a bug in a Script is a server problem. The error is already telling you which side to investigate.

#The reading order

When an error appears, don't stare at your whole script. Do this:

1. Read the location. Which script, which line. Open it.
2. Read the line. What is this line actually doing? (What variable, what call?)
3. Name the suspicious thing. Is a variable maybe nil? A string where a number belongs?
4. Add a print above it. See what the values actually are:

local coins = player.leaderstats.Coins
print("coins is:", coins, "value:", coins and coins.Value)

coins.Value = coins.Value + 10

print showing coins is: nil beats any guess. The problem isn't your math — it's that Coins was never created (wrong name, or created in the wrong PlayerAdded).

#One change, one test

The discipline that keeps debugging honest: change one thing, Play, watch that same line.

Changing three things at once is how you get errors that blame the wrong line. When the fix works, re-test the same way you reproduced it. If you can't reproduce the bug, you can't verify the fix.

local coins = player.leaderstats.Coins
-- reveal what coins is

#Common mistakes

• Changing many things at once. One change, one test. Bisect, don't shotgun.
• Guessing instead of printing. Every guess is slower than one print. Get the actual values.
• Reading only the last line of a stack trace. The top frame is the deepest cause; read upward.
• Ignoring which side the error came from. A client error and a server error are different worlds. The path tells you which.

Next: the print-debugging habit that senior developers actually use.