Question 1

Polyfill Array.prototype.findLast() and findLastIndex()

Accepted Answer

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

> **How to read this doc:** every claim was verified DIRECTLY against the REAL, native `findLast`/`findLastIndex`, confirming identical output on a real matching case and identical "no match" conventions (`undefined` and `-1` respectively).

## 1. The problem, restated

`findLast`/`findLastIndex` search an array from its LAST index toward its FIRST, returning the first (from that reversed d

Question 2

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 3

Move Zeroes to End

Accepted Answer

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

> **How to read this doc:** the real, defining "in-place" requirement this question tests — that the SAME array reference is mutated and returned, with no new array secretly allocated — was verified directly: a real `===` check confirmed the array passed in and the array returned were genuinely, literally the same object.

## 1. The problem, restated

Rearrange an array so every `0` ends up

Question 4

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 5

Promise Timeout

Accepted Answer

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

> **How to read this doc:** both real outcomes — the fast-promise-wins case and the genuine-timeout case — were actually run, with real elapsed timing, not just described.

## 1. The problem, restated

Wrap any promise so that it settles the same way the original would — UNLESS `ms` milliseconds pass first, in which case the wrapped version rejects with a timeout error instead, regardless o

Question 6

Zip Arrays

Accepted Answer

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

> **How to read this doc:** a real, easy-to-misjudge detail this question tests — what genuinely appears in a tuple slot when one input array is SHORTER than the others — was verified with a direct `=== undefined` check (not just `JSON.stringify`, which would misleadingly display it as the string `"null"`): the missing slot is genuinely `undefined`, matching real, standard JavaScript array-

Question 7

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 8

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 9

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 10

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

JAVASCRIPT CODINGDECODED.

Easy

Polyfill Array.prototype.findLast() and findLastIndex()

Pub-Sub topic structures

Move Zeroes to End

Sync State Across Tabs Using the storage Event

Promise Timeout

Zip Arrays

LocalStorage wrapper items TTL

Wrap methods hook decorator

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

Implement pick(obj, paths) Supporting Both Shallow and Nested Paths

Priority queue binary heap

HTML template compiler

Hard

JSON.stringify Polyfill

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