"undefined" returned when accessing some listed properties of File object

So just to make things clear. As user2864740 said, you need to wait for the "thumbnail" event. This would result in this code:

myDropzone.on('addedFile', function(file) {
  myDropzone.on("thumbnail", function(file){            
     console.log(file);   
     console.log(file.width); // returns width
     console.log(file['width']);  // returns width
  });
});

You could even use it in accept method like this:

myDropzone.on('accept', function(file, done){
   myDropzone.on("thumbnail", function(file){
     if(file.width != 728 && file.height != 90){
       done("Resolution is not correct (Super Banner (728x90))");
     } else {
       done();
     }
   });
}

The File.width property is a DropzoneJS extension and is not a part of the core File API; it is added later.

Dropzone adds data to the file object you can use when events fire. You can access file.width and file.height if it's an image..

If applicable the image size information is made available by the time the "thumbnail" event occurs. It is not guaranteed to be set before this event.

The documentation isn't very clear on this only alluding to "when the thumbnail has been generated", but such is the behavior of the source (see the createThumbnail/resize functions) - the image size is collected when the thumbnail is generated.

---

The initial behavior is seen because console.log (in browsers, eg. Chrome, that treat it similar to console.dir) displays the "live" object. This in turn has given enough time for the asynchronous thumbnail generation, and associated image dimension gathering, to complete before the browser displays the object's now-assgined properties in the console. (This also explains why using a timeout to read the property value works - even though such is not a reliable approach.)

On the other hand, directly accessing the file.width forces immediate evaluation of the still-not-set property, which results in undefined in the "addedFile" callback.

Add a dirty timeout will not work everytime, you will get some indefined when you will send huge files or multiples files.

I got the same headache and personnaly gave up to check width and height before uploading and check in the server side. In fact you can display dropzone errors by returning an header error:

$size = getimagesize(current($_FILES['value']['tmp_name']));

if($size[0] == 840 && $size[1] == 570){
    //perfect
}
else{
    header("HTTP/1.0 406 Not Acceptable");
    echo 'Your image must be 840x570px';
    exit();
}