Javascript to enhance parameterized builds

Check out the Active Choices Plugin it does exactly what you're looking for:

jenkins-ci.org - Active Choices Plugin
github.com - Active Choices Plugin

A Jenkins UI plugin for generating and rendering multiple valueoptions for a job parameter. The parameter options can be dynamicallygenerated from a Groovy script and can respond to changes in other jobparameters. The value options can be rendered as combo-boxes,check-boxes, radio-buttons or rich HTML. Active Choices strives toprovide in a single plugin functionality found scattered among severalpre-existing plugins and some unique capabilities that are notavailable yet.

The Active Choices plug-in provides additional Jenkinsparameter types that can be rendered as user interface (UI) controlsin job forms.

Once the plugin is installed three new parameter types becomeavailable:

• Active Choices Parameter
• Active Choices Reactive Parameter
• Active Choices Reactive Reference Parameter

Active Choices parameters allow users to select value(s) for a job parameter. Parameter values can be:

• dynamically generated (using Groovy or a Scriptler script)
• dynamically updated based on other UI parameters
• multi-valued (can have more than one value)
• rendered with a variety of UI controls, including dynamic HTML

I think you would be able to accomplish your listed example with the following dynamic parameters plugin without using javascript: https://github.com/tekante/Dynamic-Jenkins-Parameter/wiki. I have not personally used the plugin as a disclaimer. It looks like it will probably need to be built first based on what I have seen in the GitHub repo and the fact that I cannot find it on the official Jenkins plugin page.