Use the image-source modules fromNativeSource(image) method passing in the image from the promise result to get an image-source. Then use toBase64String method on the image-source instance.

import * as imageSourceModule from 'image-source';

drawingPad.getDrawing().then((res) => {        
          console.log(res);
          let x = imageSourceModule.fromNativeSource(res);
          let the64string = x.toBase64String("PNG");
       });  

Let me know if that helps 😄

Thank you.
It works.
👍

Just in case anyone is banging their head on the wall - the above code by bradmartin worked for me on android perfectly, however iOS in Nativescript Vue needed the x.toBase64String("PNG") to be lowercase as such: x.toBase64String('png')