Pass variable between two functions

• You want to create new Spreadsheet from HTML side.
• You want to put the value to the created Spreadsheet from HTML side.
• In your situation, you don't want to put writeData() in createSpreadsheet(). Namely, you want to individually use both functions.

If my understanding is correct, how about this answer? Please think of this as just one of several answers.

At first, the modification point is as follows.

Modification point:

When you run the script as follows,

google.script.run.createSpreadsheet(name);
google.script.run.writeData(data);

Function of writeData() is run before the function of createSpreadsheet() is finished, because google.script.run works by the asynchronous process. In order to avoid this, in this sample script, withSuccessHandler() is used.

About the modified script, I introduce 2 patterns. Please select one of them.

Pattern 1:

In this pattern, file ID is returned from createSpreadsheet() and the value is put to the created Spreadsheet using the file ID.

Code.gs: Google Apps Script

function createSpreadsheet(name){
  var ss = SpreadsheetApp.create(name);
  var ssID = ss.getId();
  return ssID;
}

function writeData(id, data){
  var ss = SpreadsheetApp.openById(id);
  var s = ss.getSheets()[0];
  s.getRange('A1').setValue(data);
}

index.html: HTML and Javascript

<script>
  function parse_to_backend(){
    var name = document.getElementById("user_name").value;
    var data = document.getElementById("user_data").value;
    google.script.run.withSuccessHandler((id) => {
      google.script.run.writeData(id, data);
    }).createSpreadsheet(name);
  };
</script>

Pattern 2:

In this pattern, file ID is saved by PropertiesService and the value is put to the created Spreadsheet using the file ID got from PropertiesService. In this case, the file ID is saved to PropertiesService. So the created Spreadsheet can be also used by other action of Javascript using the file ID got from PropertiesService.

Code.gs: Google Apps Script

function createSpreadsheet(name){
  var ss = SpreadsheetApp.create(name);
  var ssID = ss.getId();
  PropertiesService.getScriptProperties().setProperty("ssID", ssID);
}

function writeData(data){
  var id = PropertiesService.getScriptProperties().getProperty("ssID");
  var ss = SpreadsheetApp.openById(id);
  var s = ss.getSheets()[0];
  s.getRange('A1').setValue(data);
}

index.html: HTML and Javascript

<script>
  function parse_to_backend(){
    var name = document.getElementById("user_name").value;
    var data = document.getElementById("user_data").value;
    google.script.run.withSuccessHandler(() => {
      google.script.run.writeData(data);
    }).createSpreadsheet(name);
  };
</script>

References:

• Class google.script.run
• withSuccessHandler()
• Class PropertiesService

If I misunderstood your question and this answer was not what you want, I apologize.