File API - reader.onload is never triggered?

Question

I have multiple file inputs and want to generate preview of uploaded files for each input. I'm waiting for change on all of these inputs and then I trigger new FileReader(). Unfortunately the onload action is never triggered, what am I doing wrong?

$("body").on("change", ".file", function(event){
  showPreview(event);
})

function showPreview(event){
	console.log("showPreview started");
  
	var file = event.target.files[0];
        console.log("file loaded: ", file);
  
        var reader = new FileReader();
        console.log("reader initiated: ", reader);
  
        reader.onload = function(event) {
  	  console.log("this never works : ", event);
          console.log(event.target.result);
        };
}

label {
  margin: 0 auto;
  display: block;
  width: 200px;
  height: 100px;
}

.preview {
  display: inline-block;
  width: 50px;
  height: 50px;
  border: solid 5px red;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<label>
  <span class="preview"></span>
  <input type="file" class="file">
</label>

<label>
  <span class="preview"></span>
  <input type="file" class="file">
</label>

<label>
  <span class="preview"></span>
  <input type="file" class="file">
</label>

<label>
  <span class="preview"></span>
  <input type="file" class="file">
</label>

https://jsfiddle.net/x59gctL5/

Answer 1

You need to call readAsText() within the event handler.

See documentation

Working code:

function showPreview(event){
  // ... rest of your code

  reader.onload = function(event) {
    console.log("this never works : ", event);
    console.log(event.target.result);
  };

  reader.readAsText(file);
}