Defu is a lightweight and performant deep merge utility for JavaScript objects. It prioritizes source object properties, only applying defaults if a property is missing in the target. Unlike naive deep merges, Defu handles arrays intelligently, replacing them entirely instead of merging elements. This makes it excellent for merging configuration objects where preserving source values is paramount. Its simple API and minimal dependencies make it an efficient and reliable choice ensuring clean, predictable merges without unwanted side effects.