JIRA API: Get epic for issue

To get the epic key for an issue:

Send a request to: /issue/ISSUE-NUMBER

And look at the response body:

{
    ...,
    fields: {
        ...,
        customfield_11300: ... <- here, the epic should be listed. The number can be different
    }
}

I wanted to extract the epic name for an issue and this stumped me for a few days. The key was to realise that an epic is just a parent issue, and the epic name is the summary field of the parent issue.

So:

Step 1

Find the custom field of where the epic is stored by using the editmeta query:

https://[your-jira-hostname]/jira/rest/api/2/issue/[issue-number]/editmeta

This will yield something like below which reveals the custom field Id we need

{
  "fields": {
    <SNIP>
    "customfield_12360": {
      "required": false,
      "schema": {
        "type": "any",
        "custom": "com.pyxis.greenhopper.jira:gh-epic-link",
        "customId": 12360
      },
      "name": "Epic Link",
      "operations": [
        "set"
      ]
    }
    <SNIP>
  }
}

Step 2

Query your issue, pulling out the custom field value

https://[your-jira-hostname]/jira/rest/api/2/issue/[issue-number]?fields=customfield_12360,summary

if our issue is JIRA-34 say, this will yield something like

{
  "expand": "renderedFields,names,schema,operations,editmeta,changelog,versionedRepresentations",
  "id": "39080",
  "key": "JIRA-34",
  "fields": {
    "summary": "Write heavily upvoted answers for stack overflow",
    "customfield_12360": "JIRA-33"
  }
}

Step 3

Now we know the issue number of our epic is JIRA-33, so now query the epic...

https://[your-jira-hostname]/jira/rest/api/2/issue/JIRA-33?fields=summary

{
      "expand": "renderedFields,names,schema,operations,editmeta,changelog,versionedRepresentations",
      "id": "39080",
      "key": "JIRA-33",
      "fields": {
        "summary": "Improve StackOverflow reptuation"
      }
    }

The name of the epic for JIRA-34 is "Improve StackOverflow reptuation"

Done.