Write a function `flattenObject(obj)` that flattens a nested object into dot-notation keys. `{a: {b: 1}}` becomes `{"a.b": 1}`.

💡 Hint 1: Recursion is key — check if a value is a plain object. 💡 Hint 2: Build the key path as you recurse deeper (e.g., "a.b.c"). 💡 Hint 3: If value is an object (not null, not array), recurse. Otherwise, assign to result with the full key path. ✅ Solution: ```javascript function flattenObject(obj, prefix = "", result = {}) { for (const key in obj) { const fullKey = prefix ? `${prefix}.${key}` : key; if (typeof obj[key] === "object" && obj[key] !== null && !Array.isArray(obj[key])) { flattenObject(obj[key], fullKey, result); } else { result[fullKey] = obj[key]; } } return result; } ```