Frontend System Design: Build an Autocomplete
A structured walkthrough of the autocomplete design round: requirements, architecture, race-free fetching, caching, rendering, the ARIA combobox and metrics.
~7 min readread →
All notes
Deep-dive explanations with code you can run, sticky-note interview tips and a model answer at the end of every note.
level:
Closures, Scope & the Classic setTimeout Loop
What lexical scope and closures really are, why the var + setTimeout loop prints 3 3 3, three ways to fix it, and where closures earn their keep in real code.
~6 min readread →
TypeScript Generics, Explained Through Interview Questions
Ten generics questions, from constraints and keyof to infer, distributive conditional types and variance, each with a short answer and code you can reproduce.
~6 min readread →
The Event Loop: Microtasks vs Macrotasks
How the call stack, task queue and microtask queue fit together, where rendering happens, how async/await schedules work, and output puzzles with answers.
~7 min readread →
useEffect Pitfalls Interviewers Love to Ask
Stale closures, missing cleanup, fetch races, Strict Mode's double run, infinite loops and effects you don't need, plus how useLayoutEffect differs in timing.
~7 min readread →
The this Keyword: Four Binding Rules and One Exception
How JavaScript decides what this refers to: default, implicit, explicit and new binding, the order they win in, why arrow functions ignore them, and the bugs that follow.
~6 min readread →
Angular Change Detection, OnPush & Signals Demystified
How Angular decides when to update the DOM: the tree walk, what Zone.js did, what marks an OnPush component dirty, and how signals make zoneless apps possible.
~7 min readread →
Hoisting, the TDZ and var vs let vs const
What the engine sets up before your code runs, why var reads as undefined while let throws, how the temporal dead zone works, and why const doesn't mean immutable.
~6 min readread →
type vs interface: When It Actually Matters
Declaration merging, extends vs intersections, what only type aliases can express, and the few differences that change behavior, with a clear rule of thumb.
~5 min readread →
React Reconciliation & Keys: What Really Happens
Render vs commit, the diffing rules React relies on, why index keys break inputs, how a key resets state, and what Fiber and batching change about updates.
~7 min readread →
Prototypes & Inheritance Without the Headache
How the prototype chain resolves properties, what __proto__ and prototype each mean, what new and class do under the hood, and how to wire up inheritance by hand.
~7 min readread →
switchMap vs mergeMap vs concatMap vs exhaustMap
The four RxJS flattening operators differ in one thing: what happens when a new value arrives while the previous inner observable is still running.
~6 min readread →
Implement Debounce and Throttle From Scratch
Build debounce and throttle the way interviewers expect: timelines, this and args, cancel(), leading and trailing edges, and how to test them.
~7 min readread →
Build TypeScript's Utility Types From Scratch
Reimplement Partial, Pick, Omit, Exclude, ReturnType, Awaited and friends with mapped and conditional types, plus modifiers, key remapping and template literals.
~6 min readread →
memo, useMemo and useCallback: When They Actually Help
Why components re-render, what memo really compares, how referential equality decides everything, when memoization is wasted, and where the React Compiler fits.
~6 min readread →
Angular Dependency Injection: Providers, Scopes & inject()
How Angular finds a dependency: element and environment injectors, providedIn root, provider recipes, InjectionToken, resolution modifiers and duplicate singletons.
~7 min readread →
The Node.js Event Loop Phases, Explained
What libuv does on each turn of the loop, where nextTick, promises, setImmediate and timers really run, and how to keep CPU-heavy work from blocking everything.
~7 min readread →
The Cascade, Specificity & the Box Model
How the browser decides which CSS rule wins (origin, @layer, specificity, order), how inheritance and box-sizing work, and why z-index sometimes seems broken.
~7 min readread →
Write Promise.all (and Friends) From Scratch
Implement Promise.all with native semantics, then allSettled, race, any and a concurrency-limited pool, plus the edge cases interviewers probe.
~6 min readread →
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