js uses form form select class to implement cascading menu effect_Basic knowledge

The use example is as follows:

I recommend setting the value of each option to indicate where the user is. Which one to choose?
Use document.getElementById("select1").value; or form1.select1.value;
in javascript to get the selected value.
Use the onchange event, and the triggering condition is that the option value of the select changes.

When using the cascading menu
Create two selects, their ids are select1 and select2 respectively.
Create a trigger function javascript function for select1, select1onchange(). In this function, get the value of select1.
Look up the table to get the corresponding value of select2, and add the corresponding option for select2 to reach the level connection effect.