Race condition between Google Form submission trigger and spreadsheet population?

Question

I have a Google Form that records responses to a spreadsheet. I wrote an Apps Script that is triggered when the form is submitted, which fills in other columns in the spreadsheet based on the response.

I assume that the last row of the spreadsheet is the one that contains the response that is being recorded:

function onFormSubmit(event) {
  const form = FormApp.getActiveForm();
  const spreadsheet = SpreadsheetApp.openById(form.getDestinationId());
  const sheet = spreadsheet.getSheetByName("Parts List");
  const row = sheet.getLastRow();
  ...

This worked well in testing, but today I saw an instance where a submission occurred and the script ran on the row of the previous submission, as if the trigger executed before the new response was recorded in the spreadsheet.

Is there a way to guarantee that my script executes only after the spreadsheet is updated?

As a workaround I can record responses to the spreadsheet myself, rather than using that function of Forms.

Answer 1

From the question

Is there a way to guarantee that my script executes only after the spreadsheet is updated?

Use a on form submission installable trigger for a spreadsheet instead of using the one for a form. The spreadsheet should be the one that is linked to the form.

I assume that the last row of the spreadsheet is the one that contains the response that is being recorded:

That might not be always true. If you need to grab the row of the responses use event.range.getRow() (where event is the on form submission event from a spreadsheet installable trigger)