using the FileReader api

gistfile1.js

// only files that are added through a file input field can be read
var input = document.querySelector('input[type="file"]');

// can also handle multiple files, but i'll just use one here
var file = input.files[0];

var reader = new FileReader();
reader.onload = function(e) {
  var data = e.target.result;
  // do anything with data, which is the file that has been read
}
reader.readAsDataURL(file);