Question 1

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 2

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 3

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 4

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 5

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

Question 6

Implement Your Own Array.prototype.reduceRight()

Accepted Answer

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

> **How to read this doc:** the real, defining direction claim — that this genuinely processes the array from the END toward the START — was verified directly via real string concatenation: `["a","b","c"]` correctly produced `"cba"`, the reversed order, matching real native `reduceRight` exactly.

## 1. The problem, restated

`reduceRight` is the real, mirror-image counterpart of `reduce` —

Question 7

HTML template compiler

Accepted Answer

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

> **How to read this doc:** the real, defining efficiency property this question tests — that the expensive parsing step happens only ONCE, with the returned function cheaply REUSED across many renders — was verified directly with a real counter, confirming exactly 1 compile call across 3 separate real render invocations.

## 1. The problem, restated

A naive approach would re-scan the te

Question 8

Query String Parser

Accepted Answer

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

> **How to read this doc:** the array-reconstruction claim — that a key appearing TWICE in the query string correctly becomes an array on parse, while a key appearing ONCE stays a plain string — was verified directly through a real generate-then-parse round trip.

## 1. The problem, restated

`parseQueryString(qs)` splits a query string on `&`, then each pair on `=`, decoding both key and

Question 9

JSON schema validator

Accepted Answer

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

> **How to read this doc:** all 5 real validation scenarios below — valid data, a missing field, a wrong type, a below-minimum value, and a bad nested array item — were verified directly, each producing the exact expected error message and path.

## 1. The problem, restated

Given a schema describing an object's expected shape (`type`, `required` properties, nested `properties`, array `it

Question 10

Count Occurrences of Each Element in an Array (Frequency Counter)

Accepted Answer

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

> **How to read this doc:** this "easy-looking" question is really about choosing the right accumulator. Two failures of the naive plain-object counter were verified directly: counting the word `"constructor"` produces a STRING beginning `function Object()`, and `1` and `"1"` silently merge into one key.

## 1. The problem, restated

For every distinct value in an array, report how many t

JAVASCRIPT CODINGDECODED.

Easy

Object tree traversal

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

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

Implement groupBy(array, iteratee) — lodash style

Invert Object Keys

Implement Your Own Array.prototype.reduceRight()

Medium

HTML template compiler

Query String Parser

JSON schema validator

Count Occurrences of Each Element in an Array (Frequency Counter)

Custom Object.assign polyfill

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

Query String Generator

Custom String.prototype.trim polyfill

Hard

JSON.stringify Polyfill

Query text highlighter

JSON.parse Polyfill

Virtual DOM element mapper