You can use workflows in Jira Cloud to change, or transition, an issue through a set of logical steps. Workflows typically represent a process and contain a set of statuses and transitions.
Workflow extensions enable you to set up rules relating to the transition of an issue's status within Jira Cloud, giving you increased control over how and when an issue transitions. You can also set up post-transition rules.
You can extend workflows by adding:
Post Functions - perform automated actions after an issue transitions to a new status.
Conditions - check to make sure that a requirement is met before the transition option is available.
Validators - check to make sure that a requirement is met when a user tries to transition an issue.
It's worth noting from the outset that although you may be familiar with extensions to workflows in the Jira Server version of ScriptRunner, the Cloud infrastructure differs.