Use webhooks for posting to Slack. Works great with Google Cloud Functions and Google Cloud Scheduler. The incoming webhook for your Slack team needs to be created beforehand.

post-to-slack.js

const https = require('https');

exports.postToSlack = () => {
  
  // Read more about Slack webhooks here: https://api.slack.com/messaging/webhooks 
  const webhookPath = '/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX'
 
  const payload = {
    'channel': '#mychannel',
    'username': 'slackbot',
    'icon_emoji': ':bot:',
    'text': 'Bow to your robot overlord'
  }

  const options = {
    hostname: 'hooks.slack.com',
    path: webhookPath,
    port: 443,
    method: 'POST',
    headers: {
      'Accept': '*/*',
      'Content-Type': 'application/json'
    }
  }

  const req = https.request(options);

  req.write(JSON.stringify(payload));
  req.end();
};