var reader = new FileReader(); code example

Example: filereader javascript example

//HTML
<input type='file' accept='image/*' onchange='openFile(event)'><br>
<img id='output'>

//Javascript
var openFile = function(event) {
  var input = event.target;

  var reader = new FileReader();
  reader.onload = function(){
    var dataURL = reader.result;
    var output = document.getElementById('output');
    output.src = dataURL;
  };
  reader.readAsDataURL(input.files[0]);
}