ex.js

var reader = new FileReader()
var fileByteArray = []

reader.readAsArrayBuffer(file)
reader.onloadend = function (evt) {
if (evt.target.readyState == FileReader.DONE) {
  var arrayBuffer = evt.target.result,
    array = new Uint8Array(arrayBuffer)
  for (var i = 0; i < array.length; i++) {
    fileByteArray.push(array[i])
  }
  console.log("Yay bytes! ", fileByteArray)
}