How to clear the Resolution field when you change workflow status on specific one?

[Volodymyr]

Honestly, I can't find similar questions for my issue.
https://prnt.sc/11r7pnt
I need to do this:
When task moved from Closed status to any other status (except Resolved) the Resolution field and resolution date should be cleared, and
When task moved from Resolved status to any other status (except Closed) the Resolution field and resolution date should be cleared.
Sorry for mistakes. I try to do better with my writing skill.

[John Funk · Accepted solution]

Hi @Volodymyr  - Welcome to the Atlassian Community!

I don't use server so I don't have that version in front of me to look at. But is there a Post Function available for the Transition in the Workflow to Clear Fields? 

[Volodymyr]

Yes. I can set the post function "Clear Field Value (JSU)" and clear the Resolution field, but I need to do this only in the specified way:
When task moved from Closed status to any other status (except Resolved) the Resolution field and resolution date should be cleared, and
When task moved from Resolved status to any other status (except Closed) the Resolution field and resolution date should be cleared.
I really don't understand how to do it.

[John Funk]

Place the post function on every transition to any status other than the Closed status. 

Clear both the Resolved and Resolution fields. 

[Volodymyr]

@John Funk 
Yes. Now I understand what I need to do. Thank you!

[Nic Brough -Adaptavist- · Accepted solution]

Check each transition - the ones that "un-resolve" issues, by landing in a non-closed status, should have "clear the resolution" set as a post-function.

[Volodymyr]

I understand how to do the second part but not the first part.
Could you explain it in detailed way for me?

[Nic Brough -Adaptavist-]

Go to the workflow and edit it.  You can then click on each transition in turn to get a panel that offers tabs for conditions, validators and post-functions.  Hit the post-function tab, and you should find a list of several post-functions (indexing, storing, firing event etc), to which you can add one called "update issue field" (That's the one built into Jira, it's always available, without apps/plugins).  Select resolution and the first value for "<none>" and it'll save the function, displaying it as "clear the resolution".

[Volodymyr]

I've done it a couple of times, don't sure if that's what customer wants.
I think this option is the same as "Clear the field value". If you change a status from one to another - the field Resolution will be cleared.
For example, I have a Closed status in an issue and change it to any available. The field Resolution will be cleared except the status Resolved. The same works backwards but what if I have Design status and change it to any other except Closed, Resolved it will clear the Resolution field as well, but I need to do it only from Closed, Resolved statuses.
Can I set some a validator or a post function to check a status of an issue?

[Volodymyr]

@Nic Brough -Adaptavist- 

Sorry for misunderstanding your answers. I finally get it. It's much easier than I thought before.