bolt-js's ack(string) does not work in Socket Mode

[seratch]

The ack function that @slack/socket-mode offers does not accept a string value in the way the ack() in HTTP endpoint based bolt-js receivers do. Specifically, the following slash command handler does acknowledge a request but it does not post the "Hi there" mesage in the Slack channel.

const { App } = require("@slack/bolt");
const app = new App({
  socketMode: true,
  appToken: process.env.SLACK_APP_TOKEN,
  token: process.env.SLACK_BOT_TOKEN,
});

app.command("/hello", async ({ ack }) => {
  await ack("Hi there!");
});

(async () => {
  await app.start();
  console.log("⚡️ Bolt app is running!");
})();

The cause is the way that the send method handles the body parameter:

$ node
Welcome to Node.js v16.13.2.
Type ".help" for more information.
> const body = "Hi there!"
undefined
> const payload = { ...body };
undefined
> payload
{
  '0': 'H',
  '1': 'i',
  '2': ' ',
  '3': 't',
  '4': 'h',
  '5': 'e',
  '6': 'r',
  '7': 'e',
  '8': '!'
}
>

see: https://github.com/slackapi/node-slack-sdk/blob/%40slack/socket-mode%401.2.0/packages/socket-mode/src/SocketModeClient.ts#L424-L425

Packages:

Select all that apply:

• @slack/web-api
• @slack/rtm-api
• @slack/webhooks
• @slack/oauth
• @slack/socket-mode
• I don't know

Steps to reproduce:

See the above JS script.

Expected result:

The app's bot user post a new message "Hi there!"

Actual result:

No message is posted.