Don't write your interactive Web application in custom Javascript! The Web's Declarative nature needn't be broken just because you want two-way dynamic data instead of one-way documents on your site.
P. Pinheiro da Silva. Interactive Systems Design, Specification, and Verification, volume 1946 of Lecture Notes in Computer Science, Springer, Berlin / Heidelberg, (2001)
K. Mens, I. Michiels, and R. Wuyts. 13th International Conference on Software Engineering and Knowledge Engineering, Buenos Aires, page 136--143. Knowledge Systems Institute, (June 2001)