The select multiple HTML tag allows users to select multiple items from a list. These selected items are then passed to the action handler for the form with the same widget name. This could be a problem.
<select name=”var” multiple=”yes”>
Each selected option will arrive at the action handler as:
var=option1
var=option2
var=option3
Each option will overwrite the contents of the [...]

There are many things that you can do to debug javascript:

The easiest thing is to insert alert into the code, where you may alert the values and types of variables, function arguments, and object properties.
Use confirm to force to certain path to be taken.
Use prompt if you want to be able to cut and paste [...]