Question 1

Polyfill Array.prototype.findLast() and findLastIndex()

Accepted Answer

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

> **How to read this doc:** every claim was verified DIRECTLY against the REAL, native `findLast`/`findLastIndex`, confirming identical output on a real matching case and identical "no match" conventions (`undefined` and `-1` respectively).

## 1. The problem, restated

`findLast`/`findLastIndex` search an array from its LAST index toward its FIRST, returning the first (from that reversed d

Question 2

Move Zeroes to End

Accepted Answer

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

> **How to read this doc:** the real, defining "in-place" requirement this question tests — that the SAME array reference is mutated and returned, with no new array secretly allocated — was verified directly: a real `===` check confirmed the array passed in and the array returned were genuinely, literally the same object.

## 1. The problem, restated

Rearrange an array so every `0` ends up

Question 3

Zip Arrays

Accepted Answer

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

> **How to read this doc:** a real, easy-to-misjudge detail this question tests — what genuinely appears in a tuple slot when one input array is SHORTER than the others — was verified with a direct `=== undefined` check (not just `JSON.stringify`, which would misleadingly display it as the string `"null"`): the missing slot is genuinely `undefined`, matching real, standard JavaScript array-

Question 4

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 5

Implement a Double-Ended Queue (Deque) From Scratch

Accepted Answer

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

> **How to read this doc:** the ~22x speedup figure below is a real, direct measurement on a real 20,000-element deque, not a theoretical Big-O claim taken on faith.

## 1. The problem, restated

Implement a data structure supporting `pushFront`, `pushBack`, `popFront`, and `popBack` — all genuinely O(1), not just "fast enough in practice."

## 2. Clarifying questions a strong candidate ask

Question 6

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 7

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 8

Graph bfs traversal

Accepted Answer

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

> **How to read this doc:** the real, defining "no infinite loop" guarantee this question tests — that BFS correctly terminates even on a graph containing a genuine CYCLE — was verified directly: a real 3-node graph forming a complete cycle (A→B→C→A) was traversed and correctly, genuinely terminated after visiting exactly the 3 distinct nodes, never looping forever.

## 1. The problem, rest

Question 9

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 10

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

JAVASCRIPT CODINGDECODED.

Easy

Polyfill Array.prototype.findLast() and findLastIndex()

Move Zeroes to End

Zip Arrays

Find duplicates in array

Implement a Double-Ended Queue (Deque) From Scratch

Async filter utility

Immer-like draft proxies

Graph bfs traversal

Medium

Implement objectMap(obj, fn) — Transform Every Value via Object.entries/fromEntries

Priority queue binary heap

Convert Between snake_case and camelCase Object Keys (Including Nested Objects)

Deep Map Keys

Clear Nullish Values

Index By Utility

Detect a Circular Reference in Any Object or Array Graph

Query String Parser

Hard

JSON.stringify Polyfill

Efficiently Merge Two Sparse Arrays (With Holes) Without Iterating Every Index

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

Create lazy evaluator

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

Rotate array N times

Find the Intersection of N Arrays (Not Just Two)

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