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

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 3

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 4

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

Question 5

Two Sum — Find indices of two numbers that add to target

Accepted Answer

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

> **How to read this doc:** the real, easy-to-miss "no reusing the same element" constraint this question tests was verified directly: with input `[3,2,4]` and `target=6`, the single `3` in the array is correctly NEVER paired with itself (which would require using index `0` twice) — the real, correct answer instead correctly uses `2` and `4`.

## 1. The problem, restated

Find the indices o

Question 6

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 7

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

Accepted Answer

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

> **How to read this doc:** the real, defining "map for objects" framing this question tests was verified directly: the callback correctly received each key as its SECOND argument (`(value, key) => ...`), mirroring `Array.prototype.map`'s own familiar `(value, index)` signature, confirmed via a real, direct string-interpolation test combining both the key and the value.

## 1. The problem

Question 8

Observable map/filter operators

Accepted Answer

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

> **How to read this doc:** the defining, real property this pattern is tested on — genuine LAZINESS, where an operator like `.map()` does absolutely nothing until something actually subscribes — was verified directly with a real counter: zero calls before `subscribe()`, exactly one per emission after.

## 1. The problem, restated

Unlike a Promise (which starts running the instant it is

Question 9

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 10

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

JAVASCRIPT CODINGDECODED.

Easy

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

Graph bfs traversal

Polyfill Array.prototype.map from scratch

Custom Array.prototype.flatmap polyfill

Two Sum — Find indices of two numbers that add to target

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

Medium

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

Observable map/filter operators

Deep Map Keys

Detect a Circular Reference in Any Object or Array Graph

Implement a WeakMap-Like API Shape (and Why True Weak References Cannot Be Polyfilled)

Build a Dataloader-Style Batching Utility That Coalesces Individual Async Calls

Async map limit concurrency

Implement TimeLimitedCache (LeetCode 2622) with expiry

Hard

Create lazy evaluator

Virtual DOM element mapper

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

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