Implement a deep clone function in JavaScript without using JSON.parse/JSON.stringify. Handle circular references, Dates, Maps, Sets, and Arrays.

Implement a Promise class from scratch that follows the Promises/A+ specification, supporting then(), catch(), finally(), and Promise.all().

Implement a simple Observable class (similar to RxJS) with subscribe(), map(), filter(), and take() operators.

Implement a virtual DOM diff algorithm. Given two virtual DOM trees (old and new), generate a list of patches (operations) needed to update the real DOM.

Implement an async job scheduler that processes jobs in priority order with support for cancellation and job dependencies.

Implement a concurrency-limited async map function: given an array of items, an async worker, and a concurrency limit, process all items and return results in original order, but stop and reject early if any item throws.

Implement a custom async iterator/generator that paginates through an API, yielding items one page at a time while abstracting away the pagination cursor logic.

Implement a WeakMap-based private data pattern to fully hide state from a class's public API (an alternative to private class fields, useful for pre-ES2022 environments).

Implement a function that performs structural sharing for immutable updates to a deeply nested object at an arbitrary path, without deep-cloning the entire tree.

Implement a custom async queue that processes tasks strictly in FIFO order (one at a time), where each task can itself enqueue more tasks, and expose a way to wait until the queue is fully drained.

Implement a function that computes the maximum flow value is not required — instead implement a union-find (disjoint set) data structure with path compression and union by rank.

Implement a debounced async function wrapper where only the latest call's Promise resolves with a real result, and all earlier superseded calls resolve (not reject) with a special 'cancelled' marker rather than hanging forever.

Implement a simple reactive signal/computed system (similar to SolidJS/Vue signals): createSignal() for mutable state and createComputed() for derived values that automatically recompute and notify subscribers when dependencies change.

Implement a function that finds the shortest path between two nodes in a weighted graph using Dijkstra's algorithm.

Implement a function that solves the N-Queens problem: return all distinct board configurations where N queens can be placed on an N x N chessboard without attacking each other.

Implement a lock-free-style async mutex (mutual exclusion lock) that ensures only one async critical section runs at a time, with a lock() method returning a release function.

Implement a custom Array-like class that supports negative indexing (e.g. arr.at(-1)) and lazy evaluation of a map() chain (transformations only apply when the value is actually accessed), using Proxy.

Implement a function that serializes and deserializes a binary tree to/from a string, preserving the exact structure (including null children).

Implement an LRU cache using a doubly linked list combined with a hash map, achieving true O(1) get/put without relying on the Map's insertion-order behavior.

Implement a function that detects whether the V8 JIT-observable behavior of sparse vs dense arrays would matter: write a function demonstrating and explaining the performance difference between array holes and dense arrays.