The associated Select can support client-side dynamic updates_javascript skills

This is a small program that I have written for a long time. Associated Selects, the previous and later selects are all associated, and the previous select affects the content that follows. In order to prevent the server from re-reading, I added the client's automatic update function, so that each update does not have to read the database again. Refresh the page.

Although it is designed for a system I am working on now (can be applied to the database), but its implementation theory should be used by someone.
Now I release it for everyone to study and study. You are also welcome to point out deficiencies.
Click to download this original code

Demo (the demo cannot add data, please download the original code)

<meta http-equiv="Content-Type" content="text/html; charset=gb2312"> <title>Test</title> <b> </b> <form name="frm">Demo in progress (data cannot be added to the demo, please download the original code)<p> <select size="1" name="fy" onchange='change_select("fy")'> <option value="null" style="background-color:#EEEEFF" selected> </option> <option value="add" style="background-color:#FFEEEE">Select category--</option></select></p> </form>