Googleスプレッドシートで "=slack_id(email)" でメールアドレスからSlack IDを調べる独自関数をGoogle App Scriptで作る

gas.js

var TOKEN = PropertiesService.getScriptProperties().getProperty("slack_api_token");

function slack_id(email) {
  var user = lookupByEmailSlackUser(email);
  if (user == undefined) {
    return "該当アドレスのSlack名は見つかりませんでした"
  }
  return "@" + user.name
}

function lookupByEmailSlackUser(email) {
    var options = {
        "method": "GET",
        "contentType": "application/x-www-form-urlencoded"
    };
    var response = UrlFetchApp.fetch(encodeURI("https://slack.com/api/users.lookupByEmail?token=" + TOKEN + "&email=" + email), options);
    var content = response.getContentText("UTF-8");
    var res = JSON.parse(content);
    return res.user;
}