Tham khảo tài liệu 'javascript bible_ chapter 52', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Application Intelligent Updated Flags It happens to every active Web user all the time You visit a site periodically and never know for sure what material is new since your previous visit. Often Web page authors may flag items with New or Updated .gif images when they update those items themselves. But if you fail to visit the site over a few modification periods the only items you find flagged are those that are new as of the most recent update by the page s author. Several new items from a few weeks back may be of vital interest to you but you won t have the time to look through the whole site in search of material that is more recent than your previous visit. Even if the items display their modification dates do you remember for sure the date and time of your previous visit to the page As much as I might expect a CGI program and database on a Web site to keep track of my previous visit that really is asking a great deal of the Web site. Besides not every Web site has the wherewithal to build such a database system if it can even put up its own CGIs. After surveying the way Netscape browsers store cookie information and how time calculations are performed under Navigator 3 and later I found that a feasible alternative is to build this functionality into HTML documents and let the scripting manage the feature for users. The goal is to save in the visitor s cookie file the date and time of the previous visit to a page and then use that point as a measure against items that have an authorship timestamp in the HTML document. This goal assumes of course that the visitor uses the same computer for each visit. The Cookie Conundrum Managing the cookie situation in this application is a bit more complicated than you might think. The reason is that you have to take into account the possible ways visitors may come and go from your site while surfing the Web. You In This Chapter Temporary and persistent cookies World time calculations CGI-like intelligence 64 JavaScript .
