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

  fileInput.addEventListener('change', function(e) {
    var file = fileInput.files[0];
    var textType = /text.*/;

    if (file.type.match(textType)) {
      var reader = new FileReader();

      reader.onload = function(e) {
        fileDisplayArea.innerText = reader.result;
      }

      reader.readAsText(file);	
    } else {
      fileDisplayArea.innerText = "File not supported!"
    }
  });
}