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

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 4

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 5

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 6

Immer-like draft proxies

Accepted Answer

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

> **How to read this doc:** this doc own first implementation attempt was caught FAILING the real structural-sharing requirement during verification — it deep-cloned every nested object regardless of whether it changed, satisfying deep EQUALITY but not reference IDENTITY. The corrected version below was then verified to genuinely return the SAME object reference for every untouched field, a

Question 7

Polyfill Array.prototype.map from scratch

Accepted Answer

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

> **How to read this doc:** every real behavioral claim was verified DIRECTLY against the REAL, native `Array.prototype.map`, including the single most commonly-missed detail — that native `map` genuinely SKIPS a real hole in a sparse array, confirmed via a direct `in` operator check matching native behavior exactly.

## 1. The problem, restated

Reimplement `Array.prototype.map` — calling

Question 8

Custom Array.prototype.flatmap polyfill

Accepted Answer

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

> **How to read this doc:** the real, defining constraint this question tests — that `flatMap` flattens EXACTLY one level, never more, unlike `flat`'s own configurable depth argument — was verified directly: a callback returning a doubly-nested array (`[[n]]`) correctly left the INNER array still nested in the result, matching real native `flatMap` exactly.

## 1. The problem, restated

For

Question 9

Polyfill Function.prototype.call and apply

Accepted Answer

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

> **How to read this doc:** the real, subtle correctness detail this question specifically tests — that the internal technique used to invoke the function must leave NO trace on the real context object afterward — was verified directly: a real, live check confirmed no extra enumerable property remained on the context after `myCall` completed.

## 1. The problem, restated

Implement `.call`/

Question 10

Array.prototype.forEach Polyfill

Accepted Answer

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

> **How to read this doc:** every real behavioral claim was verified DIRECTLY against the REAL, native `Array.prototype.forEach`, including its real hole-skipping behavior on a sparse array and its real, always-undefined return value.

## 1. The problem, restated

Reimplement `forEach` — calling a callback once per PRESENT element (correctly skipping a real hole in a sparse array), passing

JAVASCRIPT CODINGDECODED.

Easy

Sync State Across Tabs Using the storage Event

Promise Timeout

Wrap methods hook decorator

Fetch wrapper auto-retry

Async filter utility

Immer-like draft proxies

Polyfill Array.prototype.map from scratch

Custom Array.prototype.flatmap polyfill

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

Priority queue binary heap

DOM event delegation target

Throttle async promise resolver

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

Hard

Microtask scheduler task runner

Infinite scroll list simulator

Flatten a Deeply Nested Array Iteratively (No Recursion, No Call-Stack Limit)

Create lazy evaluator

Build a Minimal jest.fn()-Style Spy/Mock Utility

Implement a minimal Promise/A+ (MyPromise) with then/catch

Implement an LFU (Least Frequently Used) Cache

Implement a Cancelable Promise (with CancelToken / AbortController)