callback.js

// If successful, Square Point of Sale returns the following parameters.

const clientTransactionId = "client_transaction_id";
const transactionId = "transaction_id";
// const responseUrl = new URL(
//   '/create?data={' +
//     '"transaction_id":"transaction123",' +
//     '"client_transaction_id":"40",' +
//     '"status":"ok"' +
//   '}',
//   'https://squareup.com'
// );
//If there's an error, Square Point of Sale returns the following parameters.
const errorField = "error_code";
//get the data URL and encode in JSON
function getTransactionInfo(URL) {
    var data = decodeURI(URL.searchParams.get("data"));

    console.log("data: " + data);
    var transactionInfo = JSON.parse(data);
    return transactionInfo;
  }
  // Makes a result string for success situation
function handleSuccess(transactionInfo){
  var resultString ="";

  if (clientTransactionId in transactionInfo) {
    resultString += "Client Transaction ID: " + transactionInfo[clientTransactionId] + "<br>";
  }
  if (transactionId in transactionInfo) {
    resultString += "Transaction ID: " + transactionInfo[transactionId] + "<br>";
  }
   else {
    resultString += "Transaction ID: NO CARD USED<br>";
  }
  return resultString;
}


// Makes an error string for error situation
function handleError(transactionInfo){
  var resultString ="";

  if (errorField in transactionInfo) {
    resultString += "Client Transaction ID: " + transactionInfo[clientTransactionId] + "<br>";
  }
  if (transactionId in transactionInfo) {
    resultString += "Transaction ID: " + transactionInfo[transactionId] + "<br>";
  }
   else {
    resultString += "Transaction ID: PROCESSED OFFLINE OR NO CARD USED<br>";
  }
  return resultString;
}
// Determines whether error or success based on urlParams, then prints the string
function printResponse() {
  var responseUrl = window.location.href;//orginal line is   var responseUrl = window.location.href;
  var transactionInfo = getTransactionInfo(responseUrl);
  var resultString = "";

  if (errorField in transactionInfo) {
    resultString = handleError(transactionInfo);
  } else {
    resultString = handleSuccess(transactionInfo);
  }
  document.getElementById('url').innerHTML = resultString;
  // document.getElementById('url').innerHTML = "resultString";
}

function  testString(){
  document.getElementById('url').innerHTML = "test string in url div";
}