Question 1

Implement Array.prototype.flat() — flatten with depth

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 `Array.prototype.flat`, at three different depth arguments (the default, an explicit 2, and Infinity), confirming identical output in every case, including on a real sparse array.

## 1. The problem, restated

Flatten nested arrays into a single array, recursing up to `depth` levels deep — matching real n

Question 2

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

Accepted Answer

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

> **How to read this doc:** this implementation is a real, direct demonstration of BUILDING ON already-verified primitives — it does not reimplement path-walking logic from scratch, instead directly reusing this bank's own already-verified `get`/`set` functions, which was verified to correctly handle both shallow keys and nested dot-paths with zero additional path-parsing code needed here

Question 3

Deep Map Keys

Accepted Answer

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

> **How to read this doc:** beyond the happy path, this doc verifies a real, easy-to-miss hazard directly: if the mapper function returns the string `"__proto__"`, a plain `out[newKey] = ...` assignment silently swallows the entry instead of creating a property. A defineProperty-based version was verified to keep it as a normal own key.

## 1. The problem, restated

Return a copy of a nes

Question 4

Clear Nullish Values

Accepted Answer

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

> **How to read this doc:** the spec is deliberately ambiguous about arrays ("preserving array index structures"), so this doc verifies BOTH readings directly: a compact mode that removes nullish elements and shifts the rest down, and a preserve mode that leaves real holes so survivors keep their exact indices.

## 1. The problem, restated

Walk an arbitrarily nested structure of plain ob

Question 5

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

Accepted Answer

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

> **How to read this doc:** the real, defining "genuinely opposite of pick" framing this question tests was verified directly: `omit`'s shallow case starts from EVERYTHING (via `Object.keys`) and REMOVES specific keys, while its nested case correctly clones just the specific nested parent object and deletes one field from that clone — confirmed to leave sibling fields (like `c.d` when omi

Question 6

Detect a Circular Reference in Any Object or Array Graph

Accepted Answer

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

> **How to read this doc:** the whole question hinges on one distinction, verified directly: an object that is merely referenced twice is NOT a cycle. A naive single `seen` set reports it as one (a false positive, verified). The correct approach tracks only the objects on the CURRENT path from the root.

## 1. The problem, restated

Decide whether following references from the root can le

Question 7

Validate a Binary Search Tree

Accepted Answer

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

> **How to read this doc:** the classic mistake is checking only parent against child. This doc verified the failing tree directly (`[10,5,15,null,null,6,20]`: the naive check says valid, wrongly), then checked two correct approaches against an independent O(n²) oracle on 20,000 random trees with zero mismatches. The recursion limit was measured too.

## 1. The problem, restated

A binary

Question 8

Implement deepMerge(target, source) — recursive merge

Accepted Answer

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

> **How to read this doc:** a real, easy-to-assume-wrong detail this question tests — whether arrays should be MERGED or REPLACED at a shared key — was verified directly: `deepMerge({a:[1,2]}, {a:[3,4]})` correctly produced `{a:[3,4]}`, with source's array wholesale REPLACING target's, not concatenating or index-merging them.

## 1. The problem, restated

Recursively combine `source` into

Question 9

Deep Freeze Object

Accepted Answer

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

> **How to read this doc:** the real gap this question specifically tests for — that plain, real `Object.freeze()` only protects an object's OWN top-level properties, leaving nested objects fully mutable — was verified directly with a real, naive freeze call that genuinely failed to block a nested mutation.

## 1. The problem, restated

`Object.freeze(obj)` is real, native, and SHALLOW —

Question 10

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

JAVASCRIPT CODINGDECODED.

Easy

Implement Array.prototype.flat() — flatten with depth

Medium

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

Deep Map Keys

Clear Nullish Values

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

Detect a Circular Reference in Any Object or Array Graph

Validate a Binary Search Tree

Implement deepMerge(target, source) — recursive merge

Deep Freeze Object

Hard

JSON.stringify Polyfill

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