Edit in JSFiddle

document.getElementById("filetoRead").addEventListener("change",function(){
  var file = this.files[0];

  if (file) {
      var reader = new FileReader();

      reader.onload = function (evt) {
        console.log(evt);
        document.getElementById("editor").value = evt.target.result;
      };

      reader.onerror = function (evt) {
        console.error("An error ocurred reading the file",evt);
      };

      reader.readAsText(file, "UTF-8");
    }
},false);

<textarea id="editor"></textarea><br>
<p>
 Select a file from your computer to being read as text.
</p>
<input type="file" id="filetoRead" />