How can I copy a value of a custom field in JIRA whenever it's updated? I know how to do this in a transition, the field however may be updated outside transitions.
Every change to an issue fires an "event" of some sort. You can catch these events in "listeners" and then run much the same code as you have in a transition.
I'd look at Jamie's "Script runner" plugin first - that lets you write code in a listener (or post function etc) without any of the mucking around with the addon framework or coding and compiling outside Jira. It's also free (and frankly, pretty much a standard plugin nowadays)
First question is: why you need it? There's a nice history JIRA manages for the issue, and your field values are in there .... (1)
If you still need to copy the value to another custom-field, you should take a look at plugins like JJUPIN (live fields) [paid] or (maybe) Jamie's Behaviours [free] add-on.
Edit: (1) - You can use for instance Kepler Custom Fields [free] plugin (or any calculated scripted field) and print the value from the history.
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