Mission 3 of 12
Debug & Restore the Ledger Balance System
You inherit a broken ledger app - the balances do not add up and it crashes. Read the errors, trace the logic, and restore correct balances. No AI: just you, the console, and the code.
By the end of this mission: “I can read error messages, trace logic bugs, and fix a broken app without AI.”
The tech stack for this mission
JavaScript
Git
GitHub
StackBlitzWhat you learn, checkpoint by checkpoint
Checkpoint A
Find and fix the crashes so the app loads and runs
- Read error messages and pinpoint the exact line and file that breaks
- Use the browser console and DevTools to trace runtime errors
- Fix each crash step by step until the app loads and runs
Checkpoint B
Trace and correct the balance calculation logic
- Follow the money - trace how each transaction changes the balance
- Spot the logic bugs (wrong operators, off-by-one, bad rounding)
- Correct the calculations so every running balance is accurate
Checkpoint C
Verify every balance is right, then ship the fix
- Test against known inputs to prove the balances are now correct
- Write a short note on each bug you found and how you fixed it
- Commit the fix, push to GitHub and ship the working app
AI companion mode · No AI Companion
AI OFF · you debug on your own
AI is locked for this mission. Debug from docs, DevTools and code reading only. Proves you can think without autocomplete.
What proves you’re done
Fixed repo + bug-fix notes + live link
Every mission ships something real and public. If you can’t explain it, you can’t submit it.
Reading is not building.
This mission is part of the paid 12-week programme - but the way we teach is free to try. The free trial puts you inside Sigmo, the learning portal our students use daily, with the 6 Projects in 6 Days precourse: real projects, real code, published to your own GitHub.
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