Convert Blob to binary string synchronously

I'm trying to put image in clipboard when user copies canvas selection:

So I thought the right way would be to convert canvas tu dataURL, dataURL to blob and blob to binary string.

Theoretically it should be possible to skip the blob, but I don't know why.

So this is what I did:

  function copy(event) {
    console.log("copy");
    console.log(event);

    //Get DataTransfer object
    var items = (event.clipboardData || event.originalEvent.clipboardData);
    //Canvas to blob
    var blob = Blob.fromDataURL(_this.editor.selection.getSelectedImage().toDataURL("image/png"));
    //File reader to convert blob to binary string
    var reader = new FileReader();
    //File reader is for some reason asynchronous
    reader.onloadend = function () {
      items.setData(reader.result, "image/png");
    }
    //This starts the conversion
    reader.readAsBinaryString(blob);

    //Prevent default copy operation
    event.preventDefault();
    event.cancelBubble = true;
    return false;
  }
  div.addEventListener('copy', copy);

But when the DataTransfer object is used out of the paste event thread the setData has no longer any chance to take effect.

How can I do the conversion in the same function thread?