Google Apps Script request to Xero API for trial balance - snippet for Xero Forum question

TrialBalance.gs

function getTrialBalancesWithNoDate() {
  
  // .
  // .
  // .

  fetchPublicAppData('Reports/TrialBalance', '', '') // OK
  
  // .
  // .
  // .
}

function getTrialBalancesWithDate() {
  
  // .
  // .
  // .

  fetchPublicAppData('Reports/TrialBalance', '2016-07-01', 'date') // Error - Not authorised
  
  // .
  // .
  // .
}

function getInvoices() {
  
  // .
  // .
  // .

  fetchPublicAppData('Invoices', pageNumber, 'page') // OK
  
  // .
  // .
  // .
}

function fetchPublicAppData (item, parameter, query) {  

  /* For PUBLIC APPLICATION TYPE */

  if (typeof query !== 'undefined' && query !== '') {

    query = query + '=' + parameter

  } else {

    query = ''
  }

  Log.fine('query: ' + query)

  this.loadSettings(); // get latest settings

  var method = 'GET'
  var requestURL = API_END_POINT + '/' + item 
  var oauth_signature_method = 'HMAC-SHA1'
  var oauth_timestamp =  (new Date().getTime()/1000).toFixed()
  var oauth_nonce = Utils_.generateRandomString(Math.floor(Math.random() * 50))
  var oauth_version = '1.0'

  var signBase = 'GET' + '&' + encodeURIComponent(requestURL) + '&' 

  if ((item === TRIAL_BALANCE_URL) && query !== '') { 
    signBase += encodeURIComponent(query + '&')
  }

  signBase += encodeURIComponent(
    'oauth_consumer_key=' + this.consumerKey + '&' + 
    'oauth_nonce=' + oauth_nonce + '&' + 
    'oauth_signature_method=' + oauth_signature_method + '&' + 
    'oauth_timestamp=' + oauth_timestamp + '&' + 
    'oauth_token=' + this.accessToken  + '&' +                 
    'oauth_version=' + oauth_version              
  )

  if (item === INVOICE_URL && query !== '') { 
    signBase += encodeURIComponent('&' + query)
  }

  // signBase has too many strange chars to use Log.fine(), use Logger
  Logger.log(signBase)
  // Log.fine('signBase: ' + signBase);

  var sbSigned = Utilities
    .computeHmacSignature(
      Utilities.MacAlgorithm.HMAC_SHA_1, 
      signBase, 
      encodeURIComponent(this.consumerSecret) + '&' + encodeURIComponent(this.accessTokenSecret));

  Log.fine('sbSigned: ' + sbSigned);    

  var oauth_signature = Utilities.base64Encode(sbSigned);

  Log.fine('oauth_signature: ' + oauth_signature);

  var authHeader = 
    "OAuth oauth_consumer_key=\"" + this.consumerKey + 
    "\",oauth_nonce=\"" + oauth_nonce + 
    "\",oauth_token=\"" + this.accessToken + 
    "\",oauth_signature_method=\"" + oauth_signature_method + 
    "\",oauth_timestamp=\"" + oauth_timestamp + 
    "\",oauth_version=\"" + oauth_version + 
    "\",oauth_signature=\"" + 
    encodeURIComponent(oauth_signature) + "\"";

  var headers = {"User-Agent": + this.userAgent, "Authorization": authHeader, "Accept": "application/json"};
  var options = {"headers": headers, "muteHttpExceptions": true};

  requestURL = requestURL + (query === '' ? '' : '?') + query

  Log.fine('requestURL: ' + requestURL)

  var response = UrlFetchApp.fetch(requestURL, options);

  var responseCode = response.getResponseCode();
  var responseText = response.getContentText();

  if (responseCode === 200) {

    return JSON.parse(responseText);

  } else if (responseCode === 401) {

    PropertiesService.getScriptProperties().setProperty('isConnected', 'false')
    onOpen() // Reset menu
    throw new Error('The Auth token has expired, run Xero > Settings (connect)');

  } else {

    throw new Error(responseText);
  }
    
} // fetchPublicAppData()