What is the difference between Underscore.js and vanilla JavaScript?

The difference between Underscore.js and vanilla JavaScript lies in the added utility functions and convenience that Underscore.js provides. Vanilla JavaScript means using pure JavaScript, no libraries or frameworks: using the native methods and syntax to get the job done. While JavaScript has native array and object manipulation methods, some of these can be verbose or just not have the capabilities one may be looking for regarding functional programming.
Underscore.js extends JavaScript with a library of utility functions, which makes data manipulation easier, mapping, filtering, reducing and sorting of both arrays and objects. This often can make your code with it shorter and even easier to maintain when you need to perform complex operations on your data. Although vanilla JavaScript has grown more powerful with updates, starting with ES6+, developers still use Underscore.js due to the fact that ready utilities can save them lots of time and return consistent syntax across browsers in some projects.