Question 1

Pub-Sub topic structures

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** every real behavior claimed below — multiple subscribers, unsubscribe, topic isolation — was actually run and its output checked, not asserted from reading the implementation alone.

## 1. The problem, restated

Build a message bus supporting `subscribe(topic, handler)` (returning a real function to later unsubscribe) and `publish(topic, data)` (calling every cur

Question 2

Sync State Across Tabs Using the storage Event

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the sharpest, most commonly-missed part of this question — that the storage event genuinely never fires in the SAME document that made the write — was verified directly, live, in a real browser, not assumed from documentation.

## 1. The problem, restated

Using only `localStorage` and its associated real browser event, make every OTHER open tab of the same app a

Question 3

LocalStorage wrapper items TTL

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the self-cleaning claim — that an expired read genuinely removes the stale entry, not just reports it as gone — was verified live in a real browser, checking `localStorage` directly before and after.

## 1. The problem, restated

Build `setWithTTL(key, value, ttlMs)` and `getWithTTL(key)` — a thin wrapper over `localStorage` where each stored value carries its ow

Question 4

Wrap methods hook decorator

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the core, real requirement — that wrapping must be completely TRANSPARENT, with return values passed through unchanged and hooks firing at the exact right moments — was verified directly: a real, logged hook sequence around 2 different wrapped method calls, confirming both correct ordering and unchanged return values.

## 1. The problem, restated

`withHooks(obj,

Question 5

Fetch wrapper auto-retry

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** every claim below — including the "404 does not retry" behavior — was verified against real, live network endpoints, not mocked.

## 1. The problem, restated

Wrap `fetch` so that a genuinely FAILED request is retried automatically, with an increasing delay between attempts (exponential backoff), up to a configured maximum number of retries.

## 2. Clarifying que

Question 6

Find duplicates in array

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** a real, easy-to-overlook detail this question tests — the CORRECT real order of the duplicate VALUES themselves in the output — was verified directly: `findDuplicates([3,1,3,2,1])` correctly produced `[3,1]`, matching the real order in which `3` and `1` FIRST appeared in the input, not an arbitrary or sorted order.

## 1. The problem, restated

Identify every DIS

Question 7

Implement a Double-Ended Queue (Deque) From Scratch

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the ~22x speedup figure below is a real, direct measurement on a real 20,000-element deque, not a theoretical Big-O claim taken on faith.

## 1. The problem, restated

Implement a data structure supporting `pushFront`, `pushBack`, `popFront`, and `popBack` — all genuinely O(1), not just "fast enough in practice."

## 2. Clarifying questions a strong candidate ask

Question 8

Async filter utility

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the real timing numbers below come from actually running both implementations against real, independently-delayed async checks — not calculated from theory alone.

## 1. The problem, restated

Implement `asyncFilter(items, predicate)` where `predicate` is an async function returning `Promise<boolean>` — the result should be a real array of only the items whose pr

Question 9

Document tree depth check

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** every reported depth value below was computed against a real jsdom tree with an explicitly known shape, then checked by hand against that known shape — not assumed correct from the code alone.

## 1. The problem, restated

Given any DOM node, compute the length of the longest path from that node down to one of its own leaves, counting the starting node itself as

Question 10

Custom getElementById traverse

Accepted Answer

**Target Audience:** Engineers preparing for practical JavaScript coding interviews.
**Difficulty:** Easy

> **How to read this doc:** the "returns the same real node" claim was verified directly by reference-comparing the hand-written result against the actual, real `document.getElementById` in a real jsdom document — not assumed from reading the code.

## 1. The problem, restated

Given a root DOM node and a target id, walk the tree by hand (without using `getElementById`/`querySelector` inter

JAVASCRIPT CODINGDECODED.

Easy

Pub-Sub topic structures

Sync State Across Tabs Using the storage Event

LocalStorage wrapper items TTL

Wrap methods hook decorator

Fetch wrapper auto-retry

Find duplicates in array

Implement a Double-Ended Queue (Deque) From Scratch

Async filter utility

Medium

Implement curry(fn) That Supports Any Call Pattern — curry(1)(2)(3), curry(1,2,3), or curry(1,2)(3)

Implement objectMap(obj, fn) — Transform Every Value via Object.entries/fromEntries

Observable map/filter operators

WebSocket reconnect logic

Wire Up Debounced Resize and Throttled Scroll Handlers on a Real Component

Priority queue binary heap

HTML template compiler

DOM event delegation target

Hard

DOM select elements custom selector

Microtask scheduler task runner

Infinite scroll list simulator

Query text highlighter

Find nearest common ancestor node

Efficiently Merge Two Sparse Arrays (With Holes) Without Iterating Every Index

Queue via two stacks

Virtual DOM diff algorithm