A JavaScript library is a collection of pre-written JavaScript codes that developers can use to streamline their application or website development process. This library provides pre-defined functions, methods, or classes to perform certain operations, reducing the amount of code a developer has to write. Examples of popular JavaScript libraries are jQuery, React.js, and Lodash.

Creating a JavaScript library involves essential software engineering concepts and understanding of the JavaScript language. Here is a simplified process:

1. Design your Library: Before starting to code, plan and design your library. Determine what purpose it serves and what methods and functionalities it should contain.

1. Write your Code: Start coding your library. Make sure to structure your code in a way that allows others to use it easily. Use meaningful names, add error handling and focus on keeping the code clean and understandable.

1. Testing: Write tests for your code. This helps guarantee that your code works as expected and allows you to quickly find and fix bugs.

1. Documentation: Document your library. Provide a proper documentation on how to use it, which includes how to install it, examples of how to use the methods in the library, what inputs they expect, what output they return, etc.

1. Packaging and Distribution: Package your library into a single file, which others can include in their projects with a `