Update Email Signatures in Gmail Account

update-emails.js

/**
 * Set email signature on provided email addresses
 * To ensure signatures have been successfully updated, follow the steps below:
 *
 * 1. Add comma-separated email addresses (example: 'one@domain.com', 'two@domain.com', 'three@domain.com')
 * 2. Navigate to Run > setAllSignatures
 * 3. Navigate to View > Logs
 * 4. View log output "Signature successfully updated for [email address]"
 */
function setAllSignatures() {
  var signatureTemplate = getTemplate('Signature');
  
  var allSignatures = [  
    'one@domain.com'
  ];
  
  if (allSignatures.length > 0) {
    for(var i in allSignatures) {
      setSignature(allSignatures[i], signatureTemplate);
    }
  } else {
     Logger.log("Oops, you need to specify an email address");
  }
}

/**
 *
 *
 *
 *
 *
 * Helper Functions (below) - Do Not Run 
 *
 *
 *
 *
 *
 */

/**
 * Get the proper authorization from service account
 * More information on service account at https://console.developers.google.com
 *
 * @param email {String} email address to be updated
 */
function getOAuthService(email) {
  var OAUTH2_SERVICE_ACCOUNT_PRIVATE_KEY  = '-----BEGIN PRIVATE KEY-----\nMIIE...=\n-----END PRIVATE KEY-----\n';
  var OAUTH2_SERVICE_ACCOUNT_CLIENT_EMAIL = 'email-signature-updater@email-signature-100000.iam.gserviceaccount.com';
  
  // service identifier needs to change per user
  return OAuth2.createService("Email Signature Updater - " + email)
  .setTokenUrl('https://accounts.google.com/o/oauth2/token')
  .setPrivateKey(OAUTH2_SERVICE_ACCOUNT_PRIVATE_KEY)
  .setIssuer(OAUTH2_SERVICE_ACCOUNT_CLIENT_EMAIL)
  .setSubject(email)
  .setPropertyStore(PropertiesService.getScriptProperties())
  .setScope('https://www.googleapis.com/auth/gmail.settings.basic')
  .setParam('access_type', 'offline');
}

/**
 * Reset the authorization state, so that it can be re-tested.
 */
function resetService(email) {
  getOAuthService(email).reset();
}

/**
 * Get the contents of an HTML template
 *
 * @param name {String} Name of HTML file
 * @return {String} HTML string
 */
function getTemplate(name) {
  return HtmlService.createHtmlOutputFromFile(name).getContent();
}

/**
 * Call out to Gmail API to set email signature on given address
 *
 * @param userEmail {String} email address to be updated
 * @param signature {String} HTML string used for signature template
 */
function setSignature(userEmail, signature) {
  // Reset the service so we can reuse it
  resetService(userEmail);
  
  var params = {signature: signature},
      service = getOAuthService(userEmail),
      url = 'https://content.googleapis.com/gmail/v1/users/' + encodeURIComponent(userEmail) + '/settings/sendAs/' + encodeURIComponent(userEmail) + '?alt=json';

  if (service.hasAccess()) {
    var response = UrlFetchApp.fetch(url, {
      method: 'PATCH',
      muteHttpExceptions: true,
      contentType: 'application/json',
      payload: JSON.stringify(params),
      headers: {
        'Authorization': 'Bearer ' + service.getAccessToken()
      }    
    });
    
    // error handling
    if (response.getResponseCode() !== 200) {
      Logger.log("ERROR: " + response.getContentText());
    } else {
      Logger.log("Signature successfully updated for " + userEmail + "!");
    }
    
  } else {
     Logger.log(service.getLastError());
  }
 
}