Follow the steps below to add a JavaScript file to a modern page

1. Edit your SharePoint Page & add a new webpart
2. Add Data View Plus web part to the page
   
   
3. Add any data source (use a SharePoint list as a data source)
   
   
   
4. "Switch to advanced mode" and change the Control too "Custom Control"
   
   
   
   
5. Select the cog to access the control's settings
6. Select "edit template" then In template editor, go to the "Scripts" tab
   
   
   
7. In script editor, click +Add to create new script & rename it 'LoadJS'
   
   
   
8. Click the ... icon in the script editor add the following code:
   

      var script = document.createElement('script');
      script.onload = function () {
       //do stuff with the script
      };
     script.src = 'https://kwizcom.sharepoint.com/sites/st/SiteAssets/Test1.js';;
     document.head.appendChild(script);

   

9. Go back to HTML tab and add the following code there:
   

   <div class="kw-LoadJS" data-bind="text: $component.scripts.get('LoadJS').call($component,$root,$component)"></div>