filereader API in Meteor how to read as binary string

I am trying to read the contents of a file (any format but testing with a hello.txt with contents= 'hello') and output the binary string to the console in Meteor, I'm new to the framework. Whenever I check the console there is a new entry of 'undefined'. What am I missing here?

HTML

<div>
Select a file: 
<input type="file" id="fileInput">
</div>

JS:

if (Meteor.isClient) {

    window.onload = function() {
    var fileInput = document.getElementById('fileInput');

    fileInput.addEventListener('change', function(e) {
      // Put the rest of the demo code here.
      var file = fileInput.files[0];
      var reader = new FileReader();

      reader.onload = function(e) {
      var rawData = reader.result;
      }

      var output = reader.readAsBinaryString(file); 
      console.log(output);

});
}
}