Libraries in JavaScript consist of reusable functions that allow for easier application development. In a library, each individual function is written to accomplish some type of useful task. Developers design libraries to target certain operations like handling AJAX calls, framework setup, or DOM manipulation. JavaScript libraries provide high levels of abstraction and implementation of features that have gained acceptance among users such as auto-completion, uniform cross-browser interface, graphics, and so on. Examples of some popular JavaScript libraries are JQuery, Backbone.js, and Polymer.