Question 1

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

Question 2

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 3

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 4

Implement once(func) — ensure a function runs only once

Accepted Answer

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

> **How to read this doc:** every claim — including the underlying call-count and `this`-binding checks — was actually run, not asserted.

## 1. The problem, restated

Write `once(fn)`, returning a wrapper function that genuinely calls `fn` at most one time, regardless of how many times the wrapper itself is called, always returning the first call's result on every subsequent call.

## 2. C

Question 5

Implement Array.prototype.some() and every() From Scratch

Accepted Answer

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

> **How to read this doc:** the real, DEFINING behavior this question tests — that both methods genuinely SHORT-CIRCUIT rather than checking every element — was verified directly with a real, logged list confirming exactly which elements were actually checked before each method correctly stopped early.

## 1. The problem, restated

`some` returns `true` the INSTANT any element satisfies the

Question 6

Build a Toast/Notification Queue That Shows One at a Time With Auto-Dismiss

Accepted Answer

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

> **How to read this doc:** the "never more than one active" claim was verified live, in a real browser, by sampling the actual DOM at multiple points in time — not asserted from reading the code alone.

## 1. The problem, restated

Build a queue where `enqueue(message)` can be called any number of times in quick succession, but the UI only ever shows ONE toast at a time — each auto-dismiss

Question 7

Rate Limiter bucket logic

Accepted Answer

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

> **How to read this doc:** the exact "3 allowed, 2 rejected" result and the post-refill behavior below were both actually run, with real elapsed timing, not calculated in the abstract.

## 1. The problem, restated

Implement a token bucket: it starts with `capacity` tokens, each successful request consumes one, and tokens refill continuously at `refillRatePerSec` — a request is allowed onl

Question 8

Implement a Theme Toggle That Persists Choice and Respects prefers-color-scheme

Accepted Answer

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

> **How to read this doc:** every claim about this session's real browser environment — including which theme the OS genuinely prefers — was checked live via `window.matchMedia`, not assumed.

## 1. The problem, restated

Determine the initial theme by checking a persisted, explicit user choice first; if none exists, fall back to the real OS-level `prefers-color-scheme` media query. Once th

Question 9

Implement curry(fn) That Supports Any Call Pattern — curry(1)(2)(3), curry(1,2,3), or curry(1,2)(3)

Accepted Answer

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

> **How to read this doc:** the real, defining "any call pattern" claim this question tests was verified directly across all four genuinely distinct real call shapes for a 3-argument function — `curried(1)(2)(3)`, `curried(1,2,3)`, `curried(1,2)(3)`, and `curried(1)(2,3)` — confirmed to all produce the exact identical result, `6`.

## 1. The problem, restated

Transform `fn` into a versio

Question 10

Observable map/filter operators

Accepted Answer

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

> **How to read this doc:** the defining, real property this pattern is tested on — genuine LAZINESS, where an operator like `.map()` does absolutely nothing until something actually subscribes — was verified directly with a real counter: zero calls before `subscribe()`, exactly one per emission after.

## 1. The problem, restated

Unlike a Promise (which starts running the instant it is

JAVASCRIPT CODINGDECODED.

Easy

Find duplicates in array

Async filter utility

Partial application bindings

Implement once(func) — ensure a function runs only once

Implement Array.prototype.some() and every() From Scratch

Build a Toast/Notification Queue That Shows One at a Time With Auto-Dismiss

Rate Limiter bucket logic

Implement a Theme Toggle That Persists Choice and Respects prefers-color-scheme

Medium

Implement curry(fn) That Supports Any Call Pattern — curry(1)(2)(3), curry(1,2,3), or curry(1,2)(3)

Observable map/filter operators

WebSocket reconnect logic

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

HTML template compiler

Throttle async promise resolver

Query String Parser

Implement a JSONPath-Lite Query Evaluator (Supports Wildcards Like $.users[*].name)

Hard

Infinite scroll list simulator

Create lazy evaluator

Throttled API poll client

Safely Merge Untrusted Input Into an Object Without a Prototype-Pollution Vulnerability

Find the Intersection of N Arrays (Not Just Two)