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

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 3

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 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

Array.prototype.forEach Polyfill

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.forEach`, including its real hole-skipping behavior on a sparse array and its real, always-undefined return value.

## 1. The problem, restated

Reimplement `forEach` — calling a callback once per PRESENT element (correctly skipping a real hole in a sparse array), passing

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 Array.prototype.some() and every() From Scratch

Accepted Answer

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

> **How to read this doc:** the real, DEFINING behavior this question tests — that both methods genuinely SHORT-CIRCUIT rather than checking every element — was verified directly with a real, logged list confirming exactly which elements were actually checked before each method correctly stopped early.

## 1. The problem, restated

`some` returns `true` the INSTANT any element satisfies the

Question 8

Polyfill Array.prototype.filter from scratch

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.filter`, including on a real sparse array — confirming a genuine hole is never passed to the callback and never appears in the result, matching native exactly.

## 1. The problem, restated

Build a genuinely NEW array containing only the elements for which the callback returns a truthy va

Question 9

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 10

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

JAVASCRIPT CODINGDECODED.

Easy

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

Polyfill Array.prototype.map from scratch

Custom Array.prototype.flatmap polyfill

Polyfill Function.prototype.call and apply

Array.prototype.forEach Polyfill

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

Implement Array.prototype.some() and every() From Scratch

Polyfill Array.prototype.filter from scratch

Medium

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

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

Custom Object.create polyfill

Custom Object.assign polyfill

Custom String.prototype.trim polyfill

Implement Array.prototype.includes() Handling NaN Correctly

Polyfill Array.prototype.reduce

Polyfill Function.prototype.bind (with new support)

Hard

JSON.stringify Polyfill

JSON.parse Polyfill