GAS : How to check if a file exists in a specific folder and if so go to the next iteration - google-sheets

Question

I'm writing a script that does the following :
send an email with an attachment (eg proof of payment) to people that register for our event when two criteria are met: payment has been processed, and if they have not yet received their proof of payment
make a copy of an existing file, using some variables in the new copy (this works fine) to personalize the proof of payment.
The copy of the file gets a new name (personalized).
I seem to be unable to make the script check correctly if the file already exists or not.
Who can help me out ?
function createDocument(e) {
var headers = Sheets.Spreadsheets.Values.get('SHEET ID', 'Lijst!A1:G');
var lid = Sheets.Spreadsheets.Values.get('SHEET ID', 'Lijst!A2:G');
var templateId = 'TEMPLATE ID';
for (var j = 0; j < lid.values.length; j++) {
if (lid.values[j][6] == 'ok' && lid.values[j][7] != 'Bewijs Verzonden') {
//collect the data from the member
var voornaam = lid.values[j][0];
var familienaam = lid.values[j][1];
var email = lid.values[j][2];
var aantal = lid.values[j][3];
var betaaldop = lid.values[j][5];
//Make a copy of the template file
var documentId = DriveApp.getFileById(templateId).makeCopy().getId();
var doc = DriveApp.getFileById(documentId);
var docname = 'Betalingsbewijs Reunie 2019 voor ' + familienaam + ' ' + voornaam + '.pdf';
//var filecheck = DriveApp.getFolderById('FOLDER ID');
var filecheck = DriveApp.getFilesByName(docname);
if (filecheck.hasNext()) {
Logger.log("No File Found");
} else {
//Get the document body as a variable
var body = DocumentApp.openById(documentId).getBody();
//Insert the data from spreadsheet
body.replaceText('<<Voornaam>>', voornaam)
body.replaceText('<<Naam>>', familienaam)
body.replaceText('<<Aantal deelnemers>>', aantal)
body.replaceText('<<Betaald op>>', betaaldop)
DocumentApp.openById(documentId).saveAndClose();
//move file to new folder and remove from parent folder
var file = DriveApp.getFileById(documentId);
file.getParents().next().removeFile(file);
DriveApp.getFolderById('FOLDER ID').addFile(file);
var pdfname = file.setName('Betalingsbewijs Reunie 2019 voor ' + familienaam + ' ' + voornaam + '.pdf');
var steil = "email#gmail.com";
var blob = pdfname.getBlob().getAs('application/pdf');
var subject = 'Betalingsbewijs Reunie 2019 voor ' + familienaam + ' ' + voornaam + ' met ' + aantal + 'deelnemer(s)';
var htmlBody =
"Beste " + voornaam + ", " +
"<br/>" +
"<br/>" +
"<br/>Bedankt voor uw inschrijving voor de Steil reünie op zaterdag 26 januari 2019." +
"<br/>Wij hebben uw betaling goed ontvangen." +
"<br/>Uw deelname aan de reünie is nu definitief in orde." +
"<br/>In bijlage kan u uw betalingsbewijs terugvinden in pdf." +
"<br/><br/>Indien u nog vragen heeft kan u ons steeds bereiken via reunie.steil#gmail.com." +
"<br/>Met verplegende groeten," +
"<br/>Het Steil Reünie team";
var optAdvancedArgs = {
name: "Steil Reünie 2019",
htmlBody: htmlBody,
replyTo: email,
cc: steil,
attachments: [blob]
};
GmailApp.sendEmail(email, subject, body, optAdvancedArgs);
var ConfirmationRange = "Lijst!H" + [j + 2];
SpreadsheetApp.getActiveSpreadsheet().getActiveSheet().getRange(ConfirmationRange).setValue('Bewijs Verzonden');
}
}
}
}