Sixty frontend interview questions, each with a three sentence answer, one runnable snippet, why interviewers ask it, and the follow-up traps that land after a correct first answer.

45 pages on A4, 47 on US Letter, plus the same content as one markdown file for phone reading and search. Seven sections, weighted the way real loops go: JavaScript core (12), CSS and layout (10), DOM and browser (8), React (10), performance (8), networking and security (7), testing and tooling (5).

Topics cover microtask order, closures in the var and let loops, prototype lookup, coercion, sequential versus parallel await, debounce and throttle, specificity maths, stacking contexts, margin collapse, auto-fill versus auto-fit, event delegation, reflow cost, list keys and lost state, effect cleanup timing, useReducer written from scratch, context re-render cost, long tasks and INP, hero bytes and LCP, CLS and aspect-ratio, CORS preflight rules, reflected versus stored XSS, reading a JWT without verifying it, CSP, SameSite, token storage, HTTP caching, and fake timers.

Sixty code blocks, 8 to 23 lines each, 857 lines total. 36 run under Node at build time, with printed output compared to the expected value (36 passed, 0 failed). The other 24 need a browser, a CSS engine, or React and are flagged for hand-checking. Each block ends with the output it should print.