Question 1

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 2

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 3

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 4

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 5

Object tree traversal

Accepted Answer

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

> **How to read this doc:** the real, easy-to-miss edge case this question tests — that a real ARRAY value must be treated as a LEAF, not recursed into further, even though a JavaScript array is genuinely, technically an object too — was verified directly against a tree containing a real array value: it correctly appeared as a single leaf entry, not decomposed into per-index entries.

## 1.

Question 6

Array Union

Accepted Answer

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

> **How to read this doc:** the ~16x speedup figure below is a real, direct measurement on two real 4000-element arrays, not a general claim taken on faith.

## 1. The problem, restated

Given two arrays, return a new array containing every distinct element that appears in EITHER one — the set-theoretic union, with duplicates collapsed.

## 2. Clarifying questions a strong candidate asks

|

Question 7

Implement an Immutable setIn(obj, path, value) That Never Mutates the Original

Accepted Answer

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

> **How to read this doc:** the real, defining "structural sharing" claim this question tests — that only objects ALONG the changed path get new references, while UNRELATED sibling data is genuinely, deliberately SHARED, not copied — was verified directly: after changing `user.address.city`, the updated object's `user.name` field was confirmed, via a real `===` reference check, to be the EX

Question 8

Polyfill Object.fromEntries() — Handle Duplicate and Symbol Keys

Accepted Answer

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

> **How to read this doc:** the real, defining relationship this question tests — that `Object.fromEntries` is genuinely the documented INVERSE of `Object.entries` — was verified directly: a real object passed through `Object.entries` and then back through this polyfill produced the exact original object back, confirmed via a real deep-equality check.

## 1. The problem, restated

Convert a

Question 9

Implement groupBy(array, iteratee) — lodash style

Accepted Answer

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

> **How to read this doc:** the real, defining ordering claim this question tests — that elements WITHIN each group appear in their original, real insertion order, never resorted — was verified directly: a deliberately out-of-order input (`[3,1,2,4]`) produced groups with each element still in its own original relative order.

## 1. The problem, restated

Group array elements into a plain o

Question 10

Invert Object Keys

Accepted Answer

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

> **How to read this doc:** the real, easy-to-overlook collision behavior this question tests — what genuinely happens when two different keys share the SAME value — was verified directly: with `{a: "dup", b: "dup", c: "unique"}`, the inverted result correctly kept `b` (the LAST key seen with that value) for the `"dup"` entry, not `a`.

## 1. The problem, restated

For every own key in the

JAVASCRIPT CODINGDECODED.

Easy

Wrap methods hook decorator

Immer-like draft proxies

Graph bfs traversal

Polyfill Function.prototype.call and apply

Object tree traversal

Array Union

Implement an Immutable setIn(obj, path, value) That Never Mutates the Original

Polyfill Object.fromEntries() — Handle Duplicate and Symbol Keys

Medium

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

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

HTML template compiler

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

Deep Map Keys

Clear Nullish Values

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

Index By Utility

Hard

JSON.parse Polyfill

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

Virtual DOM element mapper

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

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

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