An alert can be used to let a user determine whether to proceed with an action or not.
```javascript
function askFirst() {
// Prompt the user to edit all instances of the value
let ui = SpreadsheetApp.getUi();
let response = ui.alert('Are you sure?', 'Do you really want to do this?', ui.ButtonSet.YES_NO);
if (response == ui.Button.YES){
// your code here
}
}
```