L1–L2 · Logic and TypeScript

The bugs you can see without running anything: one wrong line, or one input nobody imagined.

The story

A subscription service bills customers on the same day each month. It has run without a complaint for a year. Then, in early March, finance notices that customers who signed up on January 31 were never billed in February, and their billing day has quietly moved to the 3rd. Months later the same thing happens to customers who signed up on August 31: no charge in September, and a new billing day of the 1st.

The code is three lines:

const next = new Date(lastBilled)
next.setMonth(next.getMonth() + 1)
return next

It's correct for 28 days of every month, which is why every test passed. setMonth doesn't clamp: January 31 plus one month is "February 31", which JavaScript rolls over to March 3 (or March 2 in a leap year).

No concurrency, no infrastructure, no attacker. Just one line and one input the author didn't picture. L1 and L2 bugs are the most common in any codebase, and the ones interviewers expect everyone to catch. The skill is building reflexes: a short list of questions you ask automatically of every line and every function.

Part 1: L1 reflexes — questions for every line

The drills below each target one family from the catalogue. Ask the reflex questions on every line.

Drill 1: Arrays

Drill 2: Boundaries and comparisons

Drill 3: References

Drill 4: Strings

Drill 5: Numbers and money

Part 2: L2 reflexes — inputs nobody pictured

L2 bugs hide in functions that are right for the example in the author's head. For every function, run through this list of inputs:

Then three more questions:

• State: is every transition allowed from every state it can be called in?
• Errors: on the failure path, what's half-done, what's swallowed, what's retried?
• Types: where does TypeScript believe something it never checked (as, !, any, JSON.parse, index lookups)?

Drill 6: Time

Drill 7: State

Drill 8: Error handling

Drill 9: TypeScript holes

Drill 10: Configuration

Drill 11: Resources

Cheat sheet

Say it in the interview

"Line 3: setMonth overflows, so a customer billed on January 31 is next billed on March 3. Any subscription started on the 29th to 31st drifts. I'd clamp to the last day of the target month, and add tests for the 31st, February in leap and non-leap years, and a DST boundary."

Go deeper

• The Bug Catalogue: the full L1 and L2 lists.
• Reading in Passes: where these reflexes fit in a 60-minute review.
• Next: L3 · Data and SQL.