I’m using Google Apps Script withSuccessHandler() and even if the logs from Logger are all correct, after returning from the function in the backend, the result is always a null.

Here’s reported the code block where I call the google.script.run.withSuccessHandler() referring to the function checkBarcode().

// Check if the vessel exists
      function checkVessel() {
        const barcode = document.getElementById("barcode").value.trim();
        barcode.toString();
        if (!barcode) {
          alert("Please scan or enter a barcode.");
          return;
        }
        
        console.log("Sending barcode to server:", barcode); // Debugging
        google.script.run.withSuccessHandler(result => {
          console.log("Result received from server:", result); // Debugging
          showForm(result);;
        }).checkBarcode(barcode)

      }

Here’s reported the function checkBarcode (also the function findBarcode used in checkBarcode).

// Check if a vessel with the barcode exists
function checkBarcode(barcode) {
  barcode.toString().trim();
  Logger.log("Checking barcode: " + barcode);

  // Search for barcode in N2 sheet
  const n2Result = findBarcode("N2", barcode);

  if (n2Result) {
    Logger.log("Barcode found in N2.");
    return result = {
      sheet: "N2",
      row: n2Result.row,
      data: n2Result.data,
    };
  }

  // Search for barcode in N2O sheet
  const n2oResult = findBarcode("N2O", barcode);

  if (n2oResult) {
    Logger.log("Barcode found in N2O.");
    return result = {
      sheet: "N2O",
      row: n2oResult.row,
      data: n2oResult.data,
    };
  }

  // If not found in any sheet
  Logger.log("Barcode not found in any sheet.");
  return result = null; // Return null explicitly if not found
}

// Find barcode in a sheet
function findBarcode(sheetName, barcode) {
  const spreadsheet = SpreadsheetApp.getActiveSpreadsheet();
  const sheet = spreadsheet.getSheetByName(sheetName);

  if (!sheet) {
    Logger.log("Sheet not found: " + sheetName);
    return null;
  }

  const data = sheet.getDataRange().getValues();
  Logger.log("Data in sheet " + sheetName + ": " + JSON.stringify(data));

  for (let i = 0; i < data.length; i++) {
    Logger.log("Comparing: " + data[i][0].toString().trim() + " with " + barcode.toString().trim());
    if (data[i][0].toString().trim() === barcode.toString().trim()) {
      return { row: i + 1, data: data[i] }; // Return row index and data
    }
  }
  return null; // Not found
}

I tried to avoid using multiple function in the .gs file but the results were the same. I also tried to convert the barcode variable in string (probably too many times) to avoid errors.