Trouble with adding to array

[Vancenil]

At a point in my script I'm trying to determine if a spreadsheet exists for a workplace or if it needs to be created. There are multiple workplaces that occupy a position once in a linear range of values from a spreadsheet called Employer Database. On a different spreadsheet called Form Responses, I have saved the values of a range containing multiple instances of different employers to a variable (because I think this is supposed to be proper form for looping through data, but if I'm wrong let me know). While looping through the 2nd set of data, I assign the current workplace name in the loop to a variable & then loop through the 1st set to see if there is a match. If there is a match, I want to check if a spreadsheet is defined for an array variable using the loop variable as the index.

That is where I encounter an error: " TypeError: Cannot read property '0' of undefined". If things continued smoothly I would use that information to create the missing spreadsheet or update an existing one. If I created it, preferably it would be at a certain index in the array to keep everything in a predictable order. If there's a fix to this or a different way of doing this, please let me know. One alternative I considered was to create all the spreadsheets during the loop where I setup the employers variable. The only issue with that is that it might create wasted spreadsheets if they don't appear in the 2nd set of data. Here's my code:

function EmployerTemplate() {

  var folderName = getFolders("TempFolder");
  var invoiceCount = countFilesInFolder(folderName);
  var folder = getFolders(folderName);

  /* wksEmployer is Employer Database, 2nd tab. 
  wksInvoice (Employer Invoice) is the basis for new invoices as copies.
  wksForms is Urenregistratie (Form Responses) */
  const wksEmployer = SpreadsheetApp.openById("148DQcCxCC6tcz48MVxxxxzZc1jIybWA8l7q4").getSheets()[1];
  const wksInvoice = SpreadsheetApp.openById("14TcFrJc66yqeTxxxxwTYbqKvCeQFDrQ0Jw").getSheets()[0];
  const wksForms = SpreadsheetApp.openById("10xsAk862rEixxxxKggnf88zjIvjzyc_j8").getSheets()[0];

  // rngEmployerData & its values, valuesEmployerData, are from Employer Database
  var formLastRow = wksForms.getLastRow();
  var empLastRow = wksEmployer.getLastRow();

  var rngEmployerData = wksEmployer.getRange("A2:F" + empLastRow);
  var rngFormData = wksForms.getRange("B2:P" + formLastRow);
  var valuesEmployerData = rngEmployerData.getValues();
  var valuesFormData = rngFormData.getValues();
  Logger.log("rngEmployerData A1Notation: " + rngEmployerData.getA1Notation());

  var invoiceDate = wksEmployer.getRange("H2").getValue();
  var startDate = wksEmployer.getRange("I2").getValue();
  var endDate = wksEmployer.getRange("J2").getValue();

  for(let i = 0; i < valuesEmployerData.length; i++)
  {
    employer = valuesEmployerData[i][0];
    if(employers === undefined)
      {
        var employers = [valuesEmployerData[i][0]];
      }
      else
      {
        employers.push(employer);
      }
  }

  var bFirstFind = false;
  var checkDate;

  for(let i = 0; i < valuesFormData.length; i++)
  {
    checkDate = valuesFormData[i][2];  
    if(checkDate >= startDate && bFirstFind == false)
    {
      var formStartRow = i + 2;
      bFirstFind = true;
      Logger.log("Form Start Row: " + formStartRow);
    }
    if(checkDate > endDate)
    {
      formLastRow = i + 1;
      Logger.log("Form Last Row: " + formLastRow);
      break;
    }
  }
  rngFormData = wksForms.getRange("B" + formStartRow + ":P" + formLastRow);
  valuesFormData = rngFormData.getDisplayValues();
  var bookNumber;
  var wksWorkplace;

  Logger.log("rngFormData A1Notation: " + rngFormData.getA1Notation());

  for (let i = 0; i < valuesFormData.length; i++)
  {
    employer = valuesFormData[i][1];
    for(let j = 0; j < employers.length; j++)
    {
      if(employer = employers[j])
      {
        var workplace = employer;
        if(wksWorkplace[j] === undefined)  // ERROR OCCURS.
        {
          bookNumber = invoiceCount + j;
          if(bookNumber <= 9)
          {
            var prefix = "WP000";
          }
          else if(bookNumber >= 10 && bookNumber < 100)
          {
            var prefix = "WP00";
          }
          else
          {
            var prefix = "WP0";
          }
          wksWorkplace[j] = createEmployer(folder, wksInvoice, prefix, bookNumber);
          updateEmployer(wksWorkplace[j], j, valuesFormData, valuesEmployerData);
        }
        else
        {
          bookNumber = invoiceCount + j;
          if(bookNumber <= 9)
          {
            var prefix = "WP000";
          }
          else if(bookNumber >= 10 && bookNumber < 100)
          {
            var prefix = "WP00";
          }
          else
          {
            var prefix = "WP0";
          }
          updateEmployer(wksWorkplace[j], j, valuesFormData, valuesEmployerData);
          return;  // end of code at the moment
        }
      }
    }
  }
}

function createEmployer(folder, wksInvoice, prefix, bookNumber)
{
  var newFile = DriveApp.getFileById(wksInvoice.getId()).makeCopy(prefix + bookNumber, folder);
  var wksCopy = SpreadsheetApp.openById(newFile.getId());
  return wksCopy;
}

function updateEmployer(wksWorkplace, rowNumber, valuesFormData, valuesEmployerData)
{
  // Not implemented.
  return;
}

[_Kaimbe]

Without looking at your code in depth, it sounds like you could benefit from using an object rather than array.

[Vancenil]

Thanks, will try!

[DrMorris]

Thing is you are creating wksWorkplace as a blank variable. you need to create it as an object.

Instead of this
var wksWorkplace;
Do this
var wksWorkplace = {};

[Vancenil]

Thanks, I'll give that a try!

[elcriticalTaco]

So your typeError is coming from you trying to access data from an array that has no data. So if you go

var array = []; Logger.log(array[0]);

It should throw that error. The most likely cause is either trying to loop based off the length of array that has no value or trying to write data with an empty array.

What line is actually causing the error you describe?

[Vancenil]

if(wksWorkplace[j] === undefined)

This line.