How to add an instance code using Javascript to add a dynamic menu _ javascript skills

When registering information, you often need to use the drop-down menu to allow users to choose from, and you want to make the selection in the first drop-down box, affecting the content of the second drop-down box. Sometimes, if no selection is made in the first drop-down box, the second drop-down box will not be displayed on the page at all, in order to present a clearer page to the user. Let's take a look at the effect:

Html source code:

The Code is as follows:

Dynamic menu change