While JavaScript, as first developed by Netscape (and later Mozilla), has long had a presence on the Web for many websites, it gained a particular pitch with the rise of the Web 2.0 era of computing, in which JavaScript, combined with multiple flavors of XML, became increasingly used for the development of user interfaces for applications, both web-based and desktop-based. JavaScript was also combined with CSS to create Dynamic web pages, which have also become popular as a more accessible alternative to Flash -based websites.

With the expanded demands for JavaScript, an easier means for programmers to develop such dynamic interfaces was needed. Thus, JavaScript libraries such as JQuery and Dojo Toolkit were developed, allowing for developers to concentrate more upon more distinctive applications of AJAX. This has led to other companies and groups, such as Microsoft and Yahoo! developing their own JavaScript-based user interface libraries, which find their way into the web applications developed by these companies.

Almost all JavaScript libraries are released under either a copycenter or copyleft license to ensure license-free distribution, usage, and modification.

Furthermore, some JavaScript libraries allow for easier ties between JavaScript and other languages, such as CSS, PHP, and Java. This is to ensure that JavaScript applications will be able to run seamlessly with other, lower-level languages."