index.js

const channel = process.env.SLACK_CHANNEL;
const token = process.env.SLACK_TOKEN;
const signingSecret = process.env.SLACK_SIGNING_SECRET;

if (!channel) {
	console.error('Set the channel using the SLACK_CHANNEL env var');
	process.exit(1);
}

if (!token) {
	console.error('Set the token using the SLACK_TOKEN env var');
	process.exit(1);
}

if (!signingSecret) {
	console.error('Set the signing secret using the SLACK_SIGNING_SECRET env var');
	process.exit(1);
}

const { App } = require('@slack/bolt');
const app = new App({
	token,
	signingSecret,
});

(async () => {
	await app.start(process.env.PORT || 3000);
	console.log(`connected, listening on port ${process.env.PORT || 3000}`);

	app.event('emoji_changed', async ({ event, context}) => {
		try {
			console.log(`got: ${JSON.stringify(event)}`);
			const action = event.subtype == 'add' ? ':heavy_plus_sign:         ' : ':heavy_minus_sign:         ';

			let emojis = [];
			event.name && emojis.push(event.name);
			event.names && emojis.push(...event.names);

			let text = `${action} ` + emojis.map(e => `:${e}:`).join(' ')
			console.log(`posting message: ${text}`);
			let result = await app.client.chat.postMessage({
				token,
				channel,
				text,
				mrkdwn: true,
			})
			console.log(event.subtype, emojis)

			// send emoji names in a separate message
			text = emojis.map(e => `:${e}: \`:${e}:\``).join("\n")
			console.log(`posting message: ${text}`);
			await app.client.chat.postMessage({
				token,
				channel,
				text,
				mrkdwn: true,
			})
		} catch (error) {
			console.error(error)
		}
	});
})();