Post new emails that have a certain label to a specified Slack channel

gmail-to-slack.gs

var slack = {
  postUrl: 'https://slack.com/api/chat.postMessage',
  token: 'xoxp-0000000000000000000000000000',
  userName: 'Postman',
};

var labelToChannel = {
  'slack-finance': '#finance',
  'slack-financial-news': '#financial-news',
  'slack-cryptocurrency': '#cryptocurrency',
};

var postMessage = function(channelId, text) {
  UrlFetchApp.fetch(slack.postUrl, {
    method: 'post',
    payload: {
      token: slack.token,
      channel: channelId,
      username: slack.userName,
      text: text,
    },
  });
}

function findAndForward(label, channel) {
  var threads = GmailApp.search('is:unread label:' + label);

  threads.forEach(function (thread) {
    var message = thread.getMessages()[0];
    var subject = message.getSubject();
    var from = message.getFrom();
    var body = message.getPlainBody();
    
    if (~from.indexOf('<')){
      from = from.match(/<(.+?@.+?)>/)[1];
    }

    postMessage(channel, [
      '*' + subject + ' (' + from + ')* ',
      body.substring(0, 4000),
    ].join('\n'));

    thread.markRead();
  });
}

function myFunction() {
  Object.keys(labelToChannel)
    .forEach(function(label) {
      findAndForward(label, labelToChannel[label]);
    });
}