send Oauth 2.0 authentication request with Google Apps Script

Question

I'm using apps script to create an interaction between a spreadsheet and a website using its API. I must first authenticate with Oauth 2.0, here is the documentation:

---

Authentication - OAuth 2.0

Authentication is required before any other API call.

POST

/oauth/token

Body :

grant_type=client_credentials

Header :

Champ	Type	Description
Authorization	String	Autorization method "Basic" followed by your public_key and private_key in your settings > developer > API combined into a string "public_key:private_key" and encoded using Base64
Content-Type	String	Must be : application/x-www-form-urlencoded

Header (example) :

```
Authorization: Basic dGVzdGNsaWVudDp0ZXN0cGFzcw==
Content-Type: application/x-www-form-urlencoded
```

---

I'm completely new to API requests, and I don't understand how to format the request, I found this post:
Send POST request in Google Apps Script with Headers and Body

And as I understand, application/x-www-form-urlencoded is by default with UrlFetchApp, so I tried:

function authentication() {
  const ENDPOINT = 'api url'
  const CLIENT_ID = 'public key'
  const CLIENT_SECRET = 'secret key'
  const TOKEN_URL = ENDPOINT + '/oauth/token'
  const HEADERS = {
    'Authorization' : 'Basic ' + CLIENT_ID + ':' + CLIENT_SECRET
  }
  const BODY = 'grant_type=client_credentials'
  const OPTIONS = {
    'method' : 'post',
    'headers' : HEADERS
  }

  let response = UrlFetchApp.fetch(TOKEN_URL + "?" + BODY,OPTIONS)

  Logger.log(response.getContentText());

}

But I get a 404 error and know an unknown error.
I guess I'm doing something wrong at least with the body but I don't understand how to format properly the request.
Can someone help me?
Thanks

Answer 1

I would suggest that you refer to this documentation, Apps script has a library that allows you to use Oauth2 you can find it here.

Here is an example:

function accessProtectedResource(url, method_opt, headers_opt) {
  var service = getOAuthService();
  var maybeAuthorized = service.hasAccess();
  if (maybeAuthorized) {

    var accessToken = service.getAccessToken();
    var method = method_opt || 'get';
    var headers = headers_opt || {};
    headers['Authorization'] =
        Utilities.formatString('Bearer %s', accessToken);
    var resp = UrlFetchApp.fetch(url, {
      'headers': headers,
      'method' : method,
      'muteHttpExceptions': true, // Prevents thrown HTTP exceptions.
    });

    var code = resp.getResponseCode();
    if (code >= 200 && code < 300) {
      return resp.getContentText("utf-8"); // Success
    } else if (code == 401 || code == 403) {
       // Not fully authorized for this action.
       maybeAuthorized = false;
    } else {
       // Handle other response codes by logging them and throwing an
       // exception.
       console.error("Backend server error (%s): %s", code.toString(),
                     resp.getContentText("utf-8"));
       throw ("Backend server error: " + code);
    }
  }

  if (!maybeAuthorized) {
    // Invoke the authorization flow using the default authorization
    // prompt card.
    CardService.newAuthorizationException()
        .setAuthorizationUrl(service.getAuthorizationUrl())
        .setResourceDisplayName("Display name to show to the user")
        .throwException();
  }
}

function getOAuthService() {
  return OAuth2.createService('SERVICE_NAME')
      .setAuthorizationBaseUrl('SERVICE_AUTH_URL')
      .setTokenUrl('SERVICE_AUTH_TOKEN_URL')
      .setClientId('CLIENT_ID')
      .setClientSecret('CLIENT_SECRET')
      .setScope('SERVICE_SCOPE_REQUESTS')
      .setCallbackFunction('authCallback')
      .setCache(CacheService.getUserCache())
      .setPropertyStore(PropertiesService.getUserProperties());
}

function authCallback(callbackRequest) {
  var authorized = getOAuthService().handleCallback(callbackRequest);
  if (authorized) {
    return HtmlService.createHtmlOutput(
      'Success! <script>setTimeout(function() { top.window.close() }, 1);</script>');
  } else {
    return HtmlService.createHtmlOutput('Denied');
  }
}

/**
 * Unauthorizes the non-Google service. This is useful for OAuth
 * development/testing.  Run this method (Run > resetOAuth in the script
 * editor) to reset OAuth to re-prompt the user for OAuth.
 */
function resetOAuth() {
  getOAuthService().reset();
}