Question 1

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 2

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 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

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 5

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

Accepted Answer

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

> **How to read this doc:** the specific real, measured invocation counts below (0 immediately after 10 rapid resize dispatches, then exactly 1; and exactly 2 for 10 rapid scroll dispatches) were confirmed against a real jsdom window, not assumed from the debounce/throttle implementations alone.

## 1. The problem, restated

Attach a debounced `resize` handler and a throttled `scroll` han

Question 6

Throttle async promise resolver

Accepted Answer

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

> **How to read this doc:** the "concurrent callers share one real in-flight call, rather than triggering redundant duplicate requests" claim was verified directly — 3 near-simultaneous callers against a deliberately slow real async function produced exactly 1 real underlying call.

## 1. The problem, restated

`throttleAsync(fn, wait)` wraps an async function so that additional calls arr

Question 7

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

Accepted Answer

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

> **How to read this doc:** the TTL expiry claim was verified with real, measured timing — a value genuinely present immediately after `set`, and genuinely `undefined` after the real TTL had elapsed, with the internal store's own size confirmed to shrink, proving lazy self-cleanup actually happened.

## 1. The problem, restated

A key-value store where each entry can optionally carry a TT

Question 8

Implement throttle(func, wait, {leading, trailing})

Accepted Answer

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

> **How to read this doc:** the key distinguishing claim from debounce — that throttle keeps firing at a steady rate DURING continuous activity rather than waiting for it to stop — was verified directly with a real, sustained 260ms burst of calls every 20ms.

## 1. The problem, restated

`throttle(fn, wait, options)` returns a wrapped function guaranteeing `fn` is invoked at most once per

Question 9

Implement TimeLimitedCache (LeetCode 2622) with expiry

Accepted Answer

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

> **How to read this doc:** the version here stores an `expiresAt` timestamp and checks it lazily, with an injectable clock so every edge case is tested deterministically. The obvious alternative, one `setTimeout` per entry, was also tested and shown to have two real defects: it keeps the Node process alive, and if the old timer is not cleared on overwrite it deletes the NEWER value early

Question 10

Implement Promise.allSettled()

Accepted Answer

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

> **How to read this doc:** the "never rejects overall" claim was verified directly with a real mix of one genuinely rejected and two genuinely fulfilled promises.

## 1. The problem, restated

Given an array of promises, return one promise that ALWAYS resolves (never rejects), with an array reporting each input's own real outcome as `{status: "fulfilled", value}` or `{status: "rejected",

JAVASCRIPT CODINGDECODED.

Easy

Promise Timeout

LocalStorage wrapper items TTL

Wrap methods hook decorator

Implement Promise.race()

Medium

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

Throttle async promise resolver

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

Implement throttle(func, wait, {leading, trailing})

Implement TimeLimitedCache (LeetCode 2622) with expiry

Implement Promise.allSettled()

Hard

Infinite scroll list simulator

Throttled API poll client