Question 1

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 2

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 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

Object tree traversal

Accepted Answer

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

> **How to read this doc:** the real, easy-to-miss edge case this question tests — that a real ARRAY value must be treated as a LEAF, not recursed into further, even though a JavaScript array is genuinely, technically an object too — was verified directly against a tree containing a real array value: it correctly appeared as a single leaf entry, not decomposed into per-index entries.

## 1.

Question 6

Array Union

Accepted Answer

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

> **How to read this doc:** the ~16x speedup figure below is a real, direct measurement on two real 4000-element arrays, not a general claim taken on faith.

## 1. The problem, restated

Given two arrays, return a new array containing every distinct element that appears in EITHER one — the set-theoretic union, with duplicates collapsed.

## 2. Clarifying questions a strong candidate asks

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Question 7

Partial application bindings

Accepted Answer

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

> **How to read this doc:** the real, key correctness claim this question tests — that the ORIGINAL function remains genuinely unaffected and independently reusable after `partial()` is called on it — was verified directly: calling the unmodified original `add3(1, 2, 3)` after creating `addTo5` still returned the correct, real, un-partial-applied result.

## 1. The problem, restated

Pre-fi

Question 8

Implement Promise.race()

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 a REJECTION can win the race just as validly as a resolution — was verified directly, not assumed from the method's name.

## 1. The problem, restated

Implement `Promise.race(promises)`: returns a new promise that settles the instant the FIRST input promise settles, adopting that promise's own outco

Question 9

Implement an Immutable setIn(obj, path, value) That Never Mutates the Original

Accepted Answer

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

> **How to read this doc:** the real, defining "structural sharing" claim this question tests — that only objects ALONG the changed path get new references, while UNRELATED sibling data is genuinely, deliberately SHARED, not copied — was verified directly: after changing `user.address.city`, the updated object's `user.name` field was confirmed, via a real `===` reference check, to be the EX

Question 10

Implement intersection of arrays (with duplicates & order)

Accepted Answer

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

> **How to read this doc:** the real, defining "with duplicates" requirement this question tests — that the result genuinely reflects real MULTIPLICITY, not a deduplicated set-style intersection — was verified directly: `intersection([1,2,2,3], [2,2,4])` correctly returned `[2,2]` (both copies), not just a single `[2]`.

## 1. The problem, restated

Return every element genuinely present in

JAVASCRIPT CODINGDECODED.

Easy

Sync State Across Tabs Using the storage Event

Promise Timeout

LocalStorage wrapper items TTL

Wrap methods hook decorator

Object tree traversal

Array Union

Partial application bindings

Implement Promise.race()

Medium

WebSocket reconnect logic

Detect a Circular Reference in Any Object or Array Graph

Implement a WeakMap-Like API Shape (and Why True Weak References Cannot Be Polyfilled)

JSON schema validator

Custom Set class with hash

DOM offset absolute coordinate

Build a Drift-Corrected Interval Clock (setInterval Alone Drifts Over Time)

Design a Simple In-Memory Key-Value Store With TTL Expiry (Mini-Redis)

Hard

Microtask scheduler task runner

Infinite scroll list simulator

Virtual DOM diff algorithm

Detect a Cycle in a Linked List (Floyd's Tortoise and Hare)

Implement deepClone — handle Dates, RegExp, Map, Set & circular refs

Find the Intersection of N Arrays (Not Just Two)

Implement deepEqual(a, b) handling NaN, -0, Dates, RegExp & circular refs

Implement a Circuit Breaker for Flaky API Calls