JavaScript Production Pitfalls — 37 bugs interviewers love to ask about

JavaScript Production Pitfalls

37 bugs interviewers love to ask about

Senior frontend interviews rarely ask what an API does. They show you code that broke in production and ask why, how you'd find it, and how you'd fix it. This workbook covers 37 of those JavaScript failures, the kind that pass code review, work on your laptop and break in front of users.

Every pitfall gets two pages:

- An infographic that shows the bug at a glance: what runs when, which line is wrong, or how the pieces connect

- A full explanation: what's wrong, why it happens, the fix with code, the follow-up question an interviewer asks next with a model answer, and a drill to run on your own codebase

What's covered (6 areas):

- Scope, closures and this: var in loops, stale closures, losing this, arrow functions as methods, accidental globals

- Async and promises: missing await, async inside forEach, request waterfalls, Promise.all failing everything, out-of-order responses, swallowed errors, microtask vs macrotask ordering, fetch with no timeout

- Data, copying and mutation: shallow copies, JSON cloning, mutating arguments, sort() surprises, floating-point money, 64-bit IDs losing precision, date parsing

- Types and coercion: == surprises, || vs ??, typeof null, NaN checks, parseInt vs Number()

- Memory and performance: listeners never removed, unbounded caches, detached DOM nodes, layout thrashing, long tasks, missing debounce

- Modules and security: circular imports, prototype pollution, innerHTML and XSS

Also inside:

✓ A pitfall map at the front showing all 37 pitfalls by area

✓ Fillable drill boxes: type in any PDF reader, or print and write

✓ A review tracker to tick off each pitfall as you master it

✓ Covers modern JavaScript (ES2024+) in current browsers and Node.js

Who it's for:

frontend and full-stack developers preparing for mid-level to senior interviews, and anyone who wants to spot these bugs before they reach production.

Companion to the JavaScript Interview Workbook: that book covers the concepts, this one covers the bugs they cause.

Format: PDF, 87 pages, instant download.