check-side-effects is a crucial tool for npm package authors aiming for predictable and tree-shakeable code. It analyzes JavaScript and TypeScript code to detect potential side effects, such as modifications to global variables or DOM manipulations. Identifying and eliminating unintentional side effects leads to smaller bundle sizes, improved performance, and increased compatibility across different environments. By ensuring pure functions, check-side-effects assists in building more maintainable and optimized libraries.