slack_button.js

'/slack': (req, res) => {
  if (req.query.code !== void 0) {
    let form = new FormData();

    form.append('code', req.query.code);
    form.append('client_id', config.slack.client_id);
    form.append('client_secret', config.slack.client_secret);

    fetch('https://slack.com/api/oauth.access', {method: 'POST', body: form, headers: form.getHeaders()}).then(res => res.json()).then(data => {
      if (data.ok) {
        stores.slack.set(data.team_id, data).then(() => res.redirect('/?added=slack'));
      } else {
        throw new Error(data.error);
      }
    }).catch(err => {
      log(String.raw`type=error, source=slackButton, message="${(err.message || err.stack || err)}", timestamp=${new Date().toISOString()}`);
      res.redirect('/?error=slack');
    })
  } else {
    res.redirect('/');
  }
}

node.js code to handle a slack button callback; using tenso, but the API is almost completely compatible with express.