277. Deep Merge Objects

Deep merge two JSON-like objects using predictable, explicit rules.

Given target and source, return a new merged object:

• merge recursively only when both values are plain objects
• otherwise, use source value (overwrite)
• arrays are replaced (not merged by index)
• inputs must not be mutated

Implement:

function deepMerge(target, source) {}

Examples

deepMerge(
  { user: { name: 'Sam' } },
  { user: { age: 20 } }
);
// { user: { name: 'Sam', age: 20 } }

deepMerge(
  { tags: ['a', 'b'] },
  { tags: ['x'] }
);
// { tags: ['x'] }