how to run javascript after clicking an action

I have an action that sets a field's value. The field is an hidden field which is responsible of only carrying the text value. In the same form, there is also an placeholder field, which is visible if the hidden field has set to a value and its content is set to display hidden field's value.

What i would like to achieve is that whenever a new value set for the placeholder to display, trigger a script. This script will be a simple animation.

Any idea how can i do this? Thanks!

Solution