Tham khảo sách 'javascript bible_ chapter 48', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Application A Lookup Table One of the first ideas that intrigued me about JavaScript was the notion of delivering CGI-like functionality along with an HTML document. Numerous small data collections on the Internet currently require CGI scripting and a back-end database engine to drive them. Of course not everyone who has information to share has access to the server environment or the expertise to implement such a solution. JavaScript provides that power and the application in this chapter demonstrates one way to implement small data collection lookups. A Serverless Database Before you get too carried away with the idea of letting JavaScript take the place of your SQL database you need to recognize several limitations that prevent JavaScript from being a universal solution. First any database that you embed into an HTML document is read-only. Although you can script an interface and lookup routines for the user no provisions are available for writing revised information back to the server if that is your intention. A second consideration is the data collection s size. Unlike databases residing on servers the entire JavaScript database or subset you define for inclusion into a single HTML document must be downloaded to the user s browser before the user can work with the data. As a point of reference think about image files. At Kbps how large an image file would you tolerate downloading Whatever that limit may be anywhere from 10 to 35K depending on your patience is what your database size limit should be. For many specialpurpose collections this is plenty of space assuming one byte per character. Unlike what happens when the user downloads an embedded image file the user doesn t see special status bar messages about your database To the browser these messages are all part of the HTML coming in with the document. In This Chapter Serverless data collection lookup Data-entry validation 12 JavaScript Applications The kind of data I m talking about here is .
