Deep Merge Objects

function deepMerge(...objects) {
  return objects.reduce((acc, obj) => {
    Object.entries(obj ?? {}).forEach(([key, val]) => {
      acc[key] =
        val && typeof val === 'object' && !Array.isArray(val)
          ? deepMerge(acc[key] ?? {}, val)
          : val;
    });
    return acc;
  }, {});
}

// Usage
const defaults = { theme: 'dark', font: { size: 14, family: 'mono' } };
const user     = { font: { size: 16 } };
console.log(deepMerge(defaults, user));
// { theme: 'dark', font: { size: 16, family: 'mono' } }