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

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 3

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 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

Implement sleep(ms) and delay with value

Accepted Answer

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

> **How to read this doc:** every claim here — including the busy-wait comparison — was actually run, not reasoned about in the abstract. Read the "Thought Process" section before the code; the reasoning is the point of this question, not just the final answer.

## 1. The problem, restated

Write `sleep(ms)`, callable as `await sleep(100)`, that pauses the CALLING async function for about `

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

Build a Dataloader-Style Batching Utility That Coalesces Individual Async Calls

Accepted Answer

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

> **How to read this doc:** the entire real point of this pattern — that MULTIPLE separate, concurrent `load()` calls collapse into exactly ONE real bulk request — was verified directly: 3 concurrent calls produced exactly 1 real `batchFn` invocation, with results correctly matched back to their own individual callers.

## 1. The problem, restated

Calling a real data-fetching function on

Question 8

Run Async Functions in Sequence, Each Using the Previous Result (Promise Waterfall)

Accepted Answer

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

> **How to read this doc:** "sequential" was verified directly — not just by checking the final numeric result, but by logging each step's own start/end and confirming a later step's start genuinely never appears before the earlier step's end.

## 1. The problem, restated

Given an array of async functions, run them ONE AFTER ANOTHER — each receiving the PREVIOUS function's resolved resul

Question 9

Implement promiseRetry(fn, retries, delay) with exponential backoff

Accepted Answer

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

> **How to read this doc:** the exact real attempt counts below — 3 for the eventually-succeeding case, and `retries + 1` for the always-failing case — were confirmed with a real counter, not assumed from the recursive structure alone.

## 1. The problem, restated

`promiseRetry(fn, retries, delayMs)` calls `fn()`; on rejection, it waits `delayMs`, then tries again — doubling the delay ea

Question 10

Implement Promise.all() from scratch

Accepted Answer

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

> **How to read this doc:** the "order is preserved regardless of completion order" claim — the single most commonly-missed detail in a naive first attempt — was verified directly with a slower promise listed BEFORE a faster one.

## 1. The problem, restated

Given an array of promises, return one promise that resolves with an array of ALL their results, in the SAME order as the input arr

JAVASCRIPT CODINGDECODED.

Easy

Promise Timeout

Wrap methods hook decorator

Async filter utility

Implement Promise.race()

Implement sleep(ms) and delay with value

Medium

Throttle async promise resolver

Build a Dataloader-Style Batching Utility That Coalesces Individual Async Calls

Run Async Functions in Sequence, Each Using the Previous Result (Promise Waterfall)

Implement promiseRetry(fn, retries, delay) with exponential backoff

Implement Promise.all() from scratch

Async reduce utility

Implement a Promise Queue (TaskQueue) with concurrency

Implement Promise.any() with AggregateError

Hard

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

Implement a Cancelable Promise (with CancelToken / AbortController)