The widespread of libraries within modern software ecosystems creates complex networks of dependencies. These dependencies are fragile to breakage, outdated, or redundancy, potentially leading to cascading issues in dependent libraries. One mitigation strategy involves reducing dependencies; libraries with zero dependencies become self-contained. This paper explores the characteristics of self-contained libraries within the NPM ecosystem. Analyzing a dataset of 2,763 NPM libraries, we found that 39.49% are self-contained. Of these self-contained libraries, 40.42% previously had dependencies that were later removed. This analysis revealed a significant trend of dependency reduction within the NPM ecosystem. The most frequently removed dependency was babel-runtime. Our investigation indicates that the primary reasons for dependency removal are concerns about the performance and the size of the dependency. Our findings illuminate the nature of self-contained libraries and their origins, offering valuable insights to guide software development practices.