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//  Enter sheet name where results will be collected
var SHEET_NAME = "Sheet1";
var CHOICES_SHEET_NAME = "Choices"; //We will use it later

var SCRIPT_PROP = PropertiesService.getScriptProperties(); // new property service

//Fetching choice options from spreadsheet
function doGet(e){
  return getDataFromSheet();
}

function doPost(e){
	return handleResponse(e);
}

function getDataFromSheet() {
  try{

  	var doc = SpreadsheetApp.openById(SCRIPT_PROP.getProperty("key"));
		var sheet = doc.getSheetByName(CHOICES_SHEET_NAME);

    var headers = sheet.getRange(2, 1, sheet.getLastRow()-1, sheet.getLastColumn()).getValues();

    var data = {}

    for (i in headers) {
      var question = headers[i][0];
      var value = headers[i][1];
      var title = headers[i][2];
      if(Array.isArray(data[question])) {
        data[question].push({
          value: value,
          title: title
        });
      }else {
        data[question] = [
          {
            value: value,
            title: title
          }
        ]
      }
    }

    return ContentService
			.createTextOutput(JSON.stringify(data))
			.setMimeType(ContentService.MimeType.JSON);
  }
  catch (e) {
    return ContentService
          .createTextOutput(JSON.stringify({"status":"error", "error": e}))
          .setMimeType(ContentService.MimeType.JSON);
  }

}

function handleResponse(e) {
	// The LockService allows you to have only one invocation of the script or portions thereof run at a time.
	// More about: http://googleappsdeveloper.blogspot.co.uk/2011/10/concurrency-and-google-apps-script.html
	var lock = LockService.getPublicLock(); //Public lock locks for any invocation of script.
	lock.waitLock(30000);

	try {
		// Alternatively, you can hard code spreadsheet here
		// eg. SpreadsheetApp.openById("1AcsuboS3xxk0kj02ACcE_j4ASb8GrxyZscTU5IM-wqc")
		var doc = SpreadsheetApp.openById(SCRIPT_PROP.getProperty("key"));
		var sheet = doc.getSheetByName(SHEET_NAME);

		var data = JSON.parse(e.postData.contents);

		var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
		var nextRow = sheet.getLastRow()+1; // get next row
		var row = [];

		for (i in headers){
			if (headers[i] == "datetime"){
				row.push(new Date());
			} 

			else if (headers[i] == "raw_data"){
				row.push(JSON.stringify(data));
			}
			
			else {
				//To support multiple choice question (checkboxes)
				if(Array.isArray(data[headers[i]])) {
					row.push(data[headers[i]].join('|'));
				}else {
					row.push(data[headers[i]]);
				}
				//TODO: Support other types of questions
			}
		}

		sheet.getRange(nextRow, 1, 1, row.length).setValues([row]);
		// return json success results
		return ContentService
			.createTextOutput(JSON.stringify({"status":"success", "row": nextRow}))
			.setMimeType(ContentService.MimeType.JSON);
	} catch (e){
		// if error return this
		return ContentService
			.createTextOutput(JSON.stringify({"status":"error", "error": e}))
			.setMimeType(ContentService.MimeType.JSON);
	} finally {
		//Release public lock from line 19
		lock.releaseLock();
	}
}

function setup() {
	var doc = SpreadsheetApp.getActiveSpreadsheet();
	SCRIPT_PROP.setProperty("key", doc.getId());
}

Gist for the article:
How to create online survey for free with SurveyJS and Google Sheets