JS Machine Coding Questions — Implementations You Must Know

Deep clone

Copy nested objects/arrays so nested values are new references.
Key insight: If value is not an object (or is null), return it. Otherwise build a new array or plain object and recurse.
Gotcha: Does not handle circular refs (infinite loop). For production, use structuredClone(obj) (built-in) or a WeakMap to detect cycles.
Gotcha: Date / Map / Set need special cases if you go beyond plain objects.

function deepClone(obj) {
  if (obj === null || typeof obj !== "object") return obj;

  const copy = Array.isArray(obj) ? [] : {};

  for (let key in obj) {
    copy[key] = deepClone(obj[key]);
  }

  return copy;
}