OAuth2 for Apps Script

OAuth2 for Apps Script is a library for Google Apps Script that provides the ability to create and authorize OAuth2 tokens as well as refresh them when they expire. This library uses Apps Script's StateTokenBuilder and /usercallback endpoint to handle the redirects.

Setup

This library is already published as an Apps Script, making it easy to include in your project. To add it to your script, do the following in the Apps Script code editor:

Click on the menu item "Resources > Libraries..."
In the "Find a Library" text box, enter the script ID 1B7FSrk5Zi6L1rSxxTDgDEUsPzlukDsi4KGuTMorsTQHhGBzBkMun4iDF and click the "Select" button.
Choose a version in the dropdown box (usually best to pick the latest version).
Click the "Save" button.

Alternatively, you can copy and paste the files in the /dist directory directly into your script project.

If you are setting explicit scopes in your manifest file, ensure that the following scope is included:

https://www.googleapis.com/auth/script.external_request

Redirect URI

Before you can start authenticating against an OAuth2 provider, you usually need to register your application with that OAuth2 provider and obtain a client ID and secret. Often a provider's registration screen requires you to enter a "Redirect URI", which is the URL that the user's browser will be redirected to after they've authorized access to their account at that provider.

For this library (and the Apps Script functionality in general) the URL will always be in the following format:

https://script.google.com/macros/d/{SCRIPT ID}/usercallback

Where {SCRIPT ID} is the ID of the script that is using this library. You can find your script's ID in the Apps Script code editor by clicking on the menu item "File > Project properties".

Alternatively you can call the service's getRedirectUri() method to view the exact URL that the service will use when performing the OAuth flow:

/**
 * Logs the redirect URI to register.
 */
function logRedirectUri() {
  var service = getService();
  Logger.log(service.getRedirectUri());
}

Usage

Using the library to generate an OAuth2 token has the following basic steps.

1. Create the OAuth2 service

The OAuth2Service class contains the configuration information for a given OAuth2 provider, including its endpoints, client IDs and secrets, etc. This information is not persisted to any data store, so you'll need to create this object each time you want to use it. The example below shows how to create a service for the Google Drive API.

function getDriveService() {
  // Create a new service with the given name. The name will be used when
  // persisting the authorized token, so ensure it is unique within the
  // scope of the property store.
  return OAuth2.createService('drive')

      // Set the endpoint URLs, which are the same for all Google services.
      .setAuthorizationBaseUrl('https://accounts.google.com/o/oauth2/auth')
      .setTokenUrl('https://accounts.google.com/o/oauth2/token')

      // Set the client ID and secret, from the Google Developers Console.
      .setClientId('...')
      .setClientSecret('...')

      // Set the name of the callback function in the script referenced
      // above that should be invoked to complete the OAuth flow.
      .setCallbackFunction('authCallback')

      // Set the property store where authorized tokens should be persisted.
      .setPropertyStore(PropertiesService.getUserProperties())

      // Set the scopes to request (space-separated for Google services).
      .setScope('https://www.googleapis.com/auth/drive')

      // Below are Google-specific OAuth2 parameters.

      // Sets the login hint, which will prevent the account chooser screen
      // from being shown to users logged in with multiple accounts.
      .setParam('login_hint', Session.getActiveUser().getEmail())

      // Requests offline access.
      .setParam('access_type', 'offline')

      // Forces the approval prompt every time. This is useful for testing,
      // but not desirable in a production application.
      .setParam('approval_prompt', 'force');
}

2. Direct the user to the authorization URL

Apps Script UI's are not allowed to redirect the user's window to a new URL, so you'll need to present the authorization URL as a link for the user to click. The URL is generated by the service, using the function getAuthorizationUrl().

function showSidebar() {
  var driveService = getDriveService();
  if (!driveService.hasAccess()) {
    var authorizationUrl = driveService.getAuthorizationUrl();
    var template = HtmlService.createTemplate(
        '<a href="<?= authorizationUrl ?>" target="_blank">Authorize</a>. ' +
        'Reopen the sidebar when the authorization is complete.');
    template.authorizationUrl = authorizationUrl;
    var page = template.evaluate();
    DocumentApp.getUi().showSidebar(page);
  } else {
  // ...
  }
}

3. Handle the callback

When the user completes the OAuth2 flow, the callback function you specified for your service will be invoked. This callback function should pass its request object to the service's handleCallback function, and show a message to the user.

function authCallback(request) {
  var driveService = getDriveService();
  var isAuthorized = driveService.handleCallback(request);
  if (isAuthorized) {
    return HtmlService.createHtmlOutput('Success! You can close this tab.');
  } else {
    return HtmlService.createHtmlOutput('Denied. You can close this tab');
  }
}

If the authorization URL was opened by the Apps Script UI (via a link, button, etc) it's possible to automatically close the window/tab using window.top.close(). You can see an example of this in the sample add-on's Callback.html.

4. Get the access token

Now that the service is authorized you can use its access token to make requests to the API. The access token can be passed along with a UrlFetchApp request in the "Authorization" header.

function makeRequest() {
  var driveService = getDriveService();
  var response = UrlFetchApp.fetch('https://www.googleapis.com/drive/v2/files?maxResults=10', {
    headers: {
      Authorization: 'Bearer ' + driveService.getAccessToken()
    }
  });
  // ...
}

Logout

To logout the user or disconnect the service, perhaps so the user can select a different account, use the reset() method:

function logout() {
  var service = getDriveService()
  service.reset();
}

Best practices

Caching

Scripts that use the library heavily should enable caching on the service, so as to not exhaust their PropertiesService quotas. To enable caching, simply add a CacheService cache when configuring the service:

return OAuth2.createService('Foo')
    .setPropertyStore(PropertiesService.getUserProperties())
    .setCache(CacheService.getUserCache())
    // ...

Make sure to select a cache with the same scope (user, script, or document) as the property store you configured.

Locking

A race condition can occur when two or more script executions are both trying to refresh an expired token at the same time. This is sometimes observed in Gmail Add-ons, where a user quickly paging through their email can trigger the same add-on multiple times.

To prevent this, use locking to ensure that only one execution is refreshing the token at a time. To enable locking, simply add a LockService lock when configuring the service:

return OAuth2.createService('Foo')
    .setPropertyStore(PropertiesService.getUserProperties())
    .setCache(CacheService.getUserCache())
    .setLock(LockService.getUserLock())
    // ...

Make sure to select a lock with the same scope (user, script, or document) as the property store and cache you configured.

Advanced configuration

See below for some features of the library you may need to utilize depending on the specifics of the OAuth provider you are connecting to. See the generated reference documentation for a complete list of methods available.

Setting the token format

OAuth services can return a token in two ways: as JSON or an URL encoded string. You can set which format the token is in with setTokenFormat(tokenFormat). There are two ENUMS to set the mode: TOKEN_FORMAT.FORM_URL_ENCODED and TOKEN_FORMAT.JSON. JSON is set as default if no token format is chosen.

Setting additional token headers

Some services, such as the FitBit API, require you to set an Authorization header on access token requests. The setTokenHeaders() method allows you to pass in a JavaScript object of additional header key/value pairs to be used in these requests.

.setTokenHeaders({
  'Authorization': 'Basic ' + Utilities.base64Encode(CLIENT_ID + ':' + CLIENT_SECRET)
});

See the FitBit sample for the complete code.

Modifying the access token payload

Some OAuth providers, such as the Smartsheet API, require you to add a hash to the access token request payloads. The setTokenPayloadHandler method allows you to pass in a function to modify the payload of an access token request before the request is sent to the token endpoint:

// Set the handler for modifying the access token request payload:
.setTokenPayloadHandler(myTokenHandler)

See the Smartsheet sample for the complete code.

Storing token-related data

Some OAuth providers return IDs and other critical information in the callback URL along with the authorization code. While it's possible to capture and store these separately, they often have a lifecycle closely tied to that of the token and it makes sense to store them together. You can use Service.getStorage() to retrieve the token storage system for the service and set custom key-value pairs.

For example, the Harvest API returns the account ID of the authorized account in the callback URL. In the following code the account ID is extracted from the request parameters and saved saved into storage.

function authCallback(request) {
  var service = getService();
  var authorized = service.handleCallback(request);
  if (authorized) {
    // Gets the authorized account ID from the scope string. Assumes the
    // application is configured to work with single accounts. Has the format
    // "harvest:{ACCOUNT_ID}".
    var scope = request.parameter['scope'];
    var accountId = scope.split(':')[1];
    // Save the account ID in the service's storage.
    service.getStorage().setValue('Harvest-Account-Id', accountId);
    return HtmlService.createHtmlOutput('Success!');
  } else {
    return HtmlService.createHtmlOutput('Denied.');
  }
}

When making an authorized request the account ID is retrieved from storage and passed via a header.

if (service.hasAccess()) {
  // Retrieve the account ID from storage.
  var accountId = service.getStorage().getValue('Harvest-Account-Id');
  var url = 'https://api.harvestapp.com/v2/users/me';
  var response = UrlFetchApp.fetch(url, {
    headers: {
      'Authorization': 'Bearer ' + service.getAccessToken(),
      'User-Agent': 'Apps Script Sample',
      'Harvest-Account-Id': accountId
    }
  });

Note that calling Service.reset() will remove all custom values from storage, in addition to the token.

Passing additional parameters to the callback function

There are occasionally cases where you need to preserve some data through the OAuth flow, so that it is available in your callback function. Although you could use the token storage mechanism discussed above for that purpose, writing to the PropertiesService is expensive and not neccessary in the case where the user doesn't start or fails to complete the OAuth flow.

As an alternative you can store small amounts of data in the OAuth2 state token, which is a standard mechanism for this purpose. To do so, pass an optional hash of parameter names and values to the getAuthorizationUrl() method:

var authorizationUrl = getService().getAuthorizationUrl({
  // Pass the additional parameter "lang" with the value "fr".
  lang: 'fr'
});

These values will be stored along-side Apps Script's internal information in the encypted state token, which is passed in the authorization URL and passed back to the redirect URI. The state token is automatically decrypted in the callback function and you can access your parameters using the same request.parameter field used in web apps:

function authCallback(request) {
  var lang = request.parameter.lang;
  // ...
}

Using service accounts

This library supports the service account authorization flow, also known as the JSON Web Token (JWT) Profile. This is a two-legged OAuth flow that doesn't require a user to visit a URL and authorize access.

One common use for service accounts with Google APIs is domain-wide delegation. This process allows a G Suite domain administrator to grant an application access to all the users within the domain. When the application wishes to access the resources of a particular user, it uses the service account authorization flow to obtain an access token. See the sample GoogleServiceAccount.gs for more information.

Using alternative grant types

Although optimized for the authorization code (3-legged) and service account (JWT bearer) flows, this library supports arbitrary flows using the setGrantType() method. Use setParam() or setTokenPayloadHandler() to add fields to the token request payload, and setTokenHeaders() to add any required headers.

The most common of these is the client_credentials grant type, which often requires that the client ID and secret are passed in the Authorization header. When using this grant type, if you set a client ID and secret using setClientId() and setClientSecret() respectively then an Authorization: Basic ... header will be added to the token request automatically, since this is what most OAuth2 providers require. If your provider uses a different method of authorization then don't set the client ID and secret and add an authorization header manually.

See the sample TwitterAppOnly.gs for a working example.

Compatibility

This library was designed to work with any OAuth2 provider, but because of small differences in how they implement the standard it may be that some APIs aren't compatible. If you find an API that it doesn't work with, open an issue or fix the problem yourself and make a pull request against the source code.

Breaking changes

Version 20 - Switched from using project keys to script IDs throughout the library. When upgrading from an older version, ensure the callback URL registered with the OAuth provider is updated to use the format https://script.google.com/macros/d/{SCRIPT ID}/usercallback.
Version 22 - Renamed Service.getToken_() to Service.getToken(), since there OAuth providers that return important information in the token response.

Troubleshooting

You do not have permission to call fetch

You are setting explicit scopes in your manifest file but have forgotten to add the https://www.googleapis.com/auth/script.external_request scope used by this library (and eventually the UrlFetchApp request you are making to an API).

BitzTrail/apps-script-oauth2