A library is a collection of code used in your projects as is. This collection is presented in a file or a set of files stored in a directory (or in a number of directories).
Libraries may optionally include API documentation which enables you to use inline documentation extracted from the source code and to view the API documentation right from WebStorm.
Libraries are protected against any kind of modification, such as direct intentional editing or update result from refactoring. Libraries are also excluded from error checks. All this allows you to save resource and improve performance and productivity.