Document and tests reply behavior

README.md

@@ -64,6 +64,7 @@ or
   env:
     SLACK_WEBHOOK_URL: ${{ secrets.SLACK_WEBHOOK_URL }}
 ```
+
 or
 
 > If the `payload` is provided it will take preference over `payload-file-path`
@@ -211,6 +212,37 @@ Please note that **the message update step does not accept a channel name.** Set
     SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
 ```
 
+#### Reply to a message
+
+If you want to post a message as a reply, you can populate the `payload` with a `"thread_ts"` field. The steps after the first message posting can have `"thread_ts": "${{ steps.slack.outputs.ts }}"` in their payload. In order to reply to an existing message.
+
+Please note that **that reply to a message does not accept a channel name.** Set a channel ID for the actions that reply to messages in thread.
+
+```yaml
+- id: slack
+  uses: slackapi/[email protected]
+  with:
+    channel-id: "CHANNEL_ID"
+    payload: |
+      {
+        "text": "Deployment started (In Progress)"
+      }
+  env:
+    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+- uses: slackapi/[email protected]
+  with:
+    # Unlike the step posting a new message, this step does not accept a channel name.
+    # Please use a channel ID, not a name here.
+    channel-id: "CHANNEL_ID"
+    payload: |
+      {
+        "thread_ts": "${{ steps.slack.outputs.ts }}"
+        "text": "Deployment finished (Completed)",
+      }
+  env:
+    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN }}
+```
+
 ### Technique 3: Slack Incoming Webhook
 
 This approach allows your GitHub Actions job to post a message to a Slack channel or direct message by utilizing [Incoming Webhooks](https://api.slack.com/messaging/webhooks).

test/slack-send-test.js

@@ -143,6 +143,53 @@ describe('slack-send', () => {
         assert.equal(firstChatArgs.text, 'who let the dogs out?', 'Correct message provided to postMessage with first comma-separated channel');
         assert.equal(secondChatArgs.text, 'who let the dogs out?', 'Correct message provided to postMessage with second comma-separated channel');
       });
+
+      it("should send a reply-message using the postMessage API if thread_ts payload field is used'", async () => {
+        fakeCore.getInput
+          .withArgs('payload')
+          .returns('{"thread_ts":"123456","text":"who let the dogs out?"}');
+        fakeCore.getInput.withArgs('channel-id').returns('C123456');
+        await slackSend(fakeCore);
+        assert.equal(
+          fakeCore.setOutput.firstCall.firstArg,
+          'ts',
+          'Output name set to ts',
+        );
+        assert.equal(
+          fakeCore.setOutput.secondCall.firstArg,
+          'thread_ts',
+          'Output name set to thread_ts',
+        );
+        assert(
+          fakeCore.setOutput.secondCall.lastArg.length > 0,
+          'Time output a non-zero-length string',
+        );
+        assert.equal(
+          fakeCore.setOutput.lastCall.firstArg,
+          'time',
+          'Output name set to time',
+        );
+        assert(
+          fakeCore.setOutput.lastCall.lastArg.length > 0,
+          'Time output a non-zero-length string',
+        );
+        const chatArgs = ChatStub.postMessage.lastCall.firstArg;
+        assert.equal(
+          chatArgs.channel,
+          'C123456',
+          'Correct channel provided to postMessage',
+        );
+        assert.equal(
+          chatArgs.thread_ts,
+          '123456',
+          'Correct thread_ts provided to postMessage',
+        );
+        assert.equal(
+          chatArgs.text,
+          'who let the dogs out?',
+          'Correct message provided to postMessage',
+        );
+      });
     });
     describe('sad path', () => {
       it('should set an error if payload cannot be JSON parsed', async () => {