Question 1

Implement dedupe / unique — remove duplicates preserving order

Accepted Answer

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

> **How to read this doc:** the real, easy-to-overlook detail this question tests — exactly which values a JavaScript `Set` considers "the same" for deduplication purposes — was verified directly: a real `NaN` was correctly deduplicated against another `NaN` (unlike `===`, which never treats two `NaN`s as equal), and `+0`/`-0` were also confirmed to collapse into a single entry.

## 1. The

Question 2

Custom Set class with hash

Accepted Answer

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

> **How to read this doc:** the real, defining test of any hash table implementation — correctly handling a COLLISION, where two different values land in the same bucket — was verified directly with a real, deliberately forced single-bucket scenario (3 values, 1 bucket), confirming all 3 remained correctly, independently trackable.

## 1. The problem, restated

A hash table maps each valu

Question 3

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

Question 4

Implement Array.prototype.includes() Handling NaN Correctly

Accepted Answer

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

> **How to read this doc:** the real, defining reason `includes` exists as its own method — that real native `indexOf` GENUINELY, PERMANENTLY cannot find `NaN` (since `NaN === NaN` is real, documented, always `false`) — was verified directly: `[1, NaN, 3].indexOf(NaN)` genuinely returned `-1`, while `includes(NaN)` correctly returned `true`.

## 1. The problem, restated

`includes` checks

Question 5

Object Difference

Accepted Answer

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

> **How to read this doc:** the spec ("differences/modifications between two objects") leaves the output shape open, so this doc picks a concrete, testable one: three flat maps keyed by dot-path — `added`, `removed`, `changed`. Two subtle claims were verified directly, not through `JSON.stringify` (which hides both): an existing key holding `undefined` is reported as removed, and `0` vs `

Question 6

Array subset check

Accepted Answer

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

> **How to read this doc:** the title gives no spec, so this doc defines two and checks both against brute force over 20,000 seeded random array pairs (0 mismatches). It also measures why the tempting one-liner is quadratic: at n = 20,000, nested `includes` took 318 ms versus 3.5 ms with a `Set`.

## 1. The problem, restated

Decide whether the first array is contained in the second. Two

Question 7

JSON.stringify Polyfill

Accepted Answer

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

> **How to read this doc:** "a basic `JSON.stringify`" hides a dozen rules, so this one implements the full contract (`toJSON`, replacer function, replacer array, `space`, cycle detection, string escaping) and was fuzzed against the native function: 150,000 random `(value, replacer, space)` triples (132,751 produced text, 8,534 produced `undefined`, 8,715 threw), the call sequence of 30,000

Question 8

Find nearest common ancestor node

Accepted Answer

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

> **How to read this doc:** all 4 real scenarios below — two leaves in different branches, a leaf and its own ancestor, a node and itself, and a leaf against another node's own ancestor — were verified directly against a real jsdom tree, confirming the exact correct real element every time.

## 1. The problem, restated

Given two DOM nodes, find the single, deepest node that is an ancestor

Question 9

JSON.parse Polyfill

Accepted Answer

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

> **How to read this doc:** the parser was fuzzed against the native `JSON.parse` until nothing disagreed: 813,615 short strings from a structural alphabet (every string up to length 5), every one of the 65,536 UTF-16 code units in three positions, 60,000 random documents, 240,000 random corruptions of them, and six different revivers over 20,000 documents each. Zero mismatches, on both the

Question 10

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

Accepted Answer

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

> **How to read this doc:** equality has no single right answer, so this doc first pins the semantics down, then checks them. `deepEqual` was compared with Node's `util.isDeepStrictEqual` on 240,000 pairs (each of 60,000 random graphs compared with itself, with its clone, with a clone that had exactly one change, and with an unrelated graph): 148,923 equal, 91,077 unequal, and ONE disagreem

JAVASCRIPT CODINGDECODED.

Easy

Implement dedupe / unique — remove duplicates preserving order

Medium

Custom Set class with hash

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

Implement Array.prototype.includes() Handling NaN Correctly

Object Difference

Array subset check

Hard

JSON.stringify Polyfill

Find nearest common ancestor node

JSON.parse Polyfill

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