A new method is invented to assemble data into a data set, such as item list in an Internet shopping cart, on client side that is necessary for a transaction between two parties on the Internet server side and client side. The method is to arrange application logic and data access logic on both client side and server side according to execution efficiency to achieve optimal implementations of transactions. A web browser with a build-in engine that can execute client side application program, such as Netscape Navigator 3.0 or above, is needed to implement the method. Web pages used in the method are specifically designed in which client side application program written with script language, such as JavaScript, is embedded. Also a frame in a window, which is preferred, or a new window is created on client side as a data assembling monitor window. In addition, data needed for a transaction in web pages can be retrieved dynamically and individually and assembled with necessary user input in the monitor window by the client side application program actuated by users. In an assembling process, all or at least first two of following editing operations of adding, deleting, updating, entry check, calculation, and backup, are executed on client side until final submission, save, or print. An Internet Client Side Shopping Cart is used to illustrate how to implement this method by using HTML, JavaScript, and CGI. This method can also be used on a local area network and a metropolitan area network.