Example JXA script to perform mail actions. Place the script in ~/Library/Application Scripts/com.apple.mail/.

MailAction.scpt

function performMailActionWithMessages(messages, options) {
	// do stuff, e.g.:
	const app = Application('System Events');
	app.includeStandardAdditions = true;
	
	// messages are provided as a list
	app.displayDialog(messages.map((message) => message.subject()).join(', '))
	
	// options are provided as an object with two properties: inMailboxes and forRule
	// both options can be undefined
	if (options.inMailboxes) app.displayDialog(options.inMailboxes.map((mailbox) => mailbox.name()).join(', '))
	if (options.forRule) app.displayDialog(options.forRule.name())
}