deepmerge is a popular and versatile JavaScript library for deeply merging JavaScript objects. Unlike simple object assigns, deepmerge recursively merges nested objects and arrays, ensuring that values are combined rather than overwritten. It provides fine-grained control with options to customize the merging behavior, handling arrays, prototype pollution, and circular references. Widely used in configuration management and state management scenarios, deepmerge offers a robust solution for complex object merging needs while maintaining data integrity.