send direct messages (including group DMs) from GAS

sendDirectMessage.gs

const token = 'xoxb-xxx'

function main() {
  var message = 'Hi :)'
  
  // send direct message
  var userID = 'UX...'
  sendMessage(userID, message)
  
  // send group direct message
  var userIDs = ['UX...', 'OX...']
  var channelID = findOrCreateChannel(userIDs)
  if (channelID) {
    sendMessage(channelID, message)
  }
}

function findOrCreateChannel(userIDs) {
  if (userIDs.length == 0) {
    return null
  }
  var params = {
    'users': userIDs.join(',')
  }
  var options = {
    'headers': {
      'Authorization': `Bearer ${token}`
    },
    'method': 'post',
    'contentType': 'application/json',
    'payload': JSON.stringify(params)
  }
  var response = UrlFetchApp.fetch('https://slack.com/api/conversations.open', options)
  if (response.getResponseCode() >= 400) {
    return nil
  }
  var json = JSON.parse(response.getContentText())
  return json.channel.id
}

function sendMessage(channelID, message) {
  var params = {
    'channel': channelID,
    'text': message
  }
  var options = {
    'headers': {
      'Authorization': `Bearer ${token}`
    },
    'method': 'post',
    'contentType': 'application/json',
    'payload': JSON.stringify(params)
  }
  UrlFetchApp.fetch('https://slack.com/api/chat.postMessage', options)
}