Create a Multi-Select Drop-Down for any cell in Google Sheets

GSheets-Multi-Select-Drop-Down.gs

const separator = ', ';
const dvType = SpreadsheetApp.DataValidationCriteria.VALUE_IN_RANGE;

function onEdit(e) {
  if (e.value != null && e.oldValue != null && e.value !== "") {
    var dataValidation = e.range.getDataValidation();
    if(dataValidation != null && dataValidation.getCriteriaType() == dvType && 
      e.value.indexOf(separator) < 0 && e.oldValue.indexOf(e.value) < 0) {
        e.range.setValue(e.oldValue + separator + e.value);
    }
  }
}

Based on https://spreadsheetpoint.com/multiple-selection-drop-down-google-sheets/

Improvements:

• Works on any cell, only depends on which cell was edited (not on which spreadsheet or cell is currently active)
• Only proceeds if the edit was on a single cell and that the cell wasn't blanked out (aka reset)
• Checks for if the cell has "data validation" enabled for "list from a range"
• Only proceeds if the new value itself isn't just an edit of the existing value
• Uses a global 'separator' variable for easy modification

Hello sir, just a question. How should the code go if you only want this to work on a specific tab? Thank you

I got the following error

TypeError: Cannot read properties of undefined (reading 'value')
onEdit @ Code.gs:4

does this have something to do with validating e.value and e.oldValue? how do I do that?

I have the same error! :c

edit: i realized that it throws the error in the AppScript editor, but it's executing the script when I make edits in the spreadsheet (based on the execution history). However, it still doesn't seem to be doing anything on the sheet (the original script from spreadsheetpoint is working though).