I think you're getting Javascript libraries confused with Javascript frameworks. Libraries have come and gone, yes, but there is a clear standard now in jQuery,

I think you're getting Javascript libraries confused with Javascript frameworks. Libraries have come and gone, yes, but there is a clear standard now in jQuery, and as such it's safe to assume it can be included in a stack for a long term project. Frameworks for building complex Javascript applications, however, are relatively new to the scene. One of the major offerings will eventually become the jQuery of Javascript frameworks and become the de facto standard, safe to include in a stack for a long term project as well.