【GAS】WebページのスクリーンショットをSlackに自動で送信する方法

function sendScreenshotOfWebPage() {
  const TOKEN         = '…';
  const WEB_PAGE_URL  = 'http://sample.com/';
  const TITLE         = 'サンプル';
  const WIDTH         = 800;
  const HEIGHT        = 600;
  const CHANNEL       = '#general';
  const CAPTURING_URL = 
      'https://s.wordpress.com/mshots/v1/' 
    + WEB_PAGE_URL
    + '?w=' + WIDTH
    + '&h=' + HEIGHT
  ;
  
  const RESPONSE_DATA = UrlFetchApp.fetch( CAPTURING_URL );
  const RESPONSE_BLOB = RESPONSE_DATA.getBlob();
  
  sendImage( TOKEN, RESPONSE_BLOB, TITLE, CHANNEL );
}

function sendImage( token, image, title, channel ) {
  const PAYLOAD = {
    'token'   : token,
    'file'    : image,
    'channels': channel,
    'title'   : title,
  };
  
  const OPTIONS = { 'method' : 'post', 'payload': PAYLOAD };
  UrlFetchApp.fetch( 'https://slack.com/api/files.upload', OPTIONS );
}