Upload File as String to JavaScript Variable

This variation on yent's answer manages multiple uploads and uses jquery:

HTML:

<form id="myform">
  <p>
    <input id="myfile" name="files[]" multiple="" type="file" />
    <textarea id="text" rows="20" cols="40">nothing loaded</textarea>
  </p>
</form>

script:

$("#myfile").on("change", function (changeEvent) {
  for (var i = 0; i < changeEvent.target.files.length; ++i) {
    (function (file) {               // Wrap current file in a closure.
      var loader = new FileReader();
      loader.onload = function (loadEvent) {
        if (loadEvent.target.readyState != 2)
          return;
        if (loadEvent.target.error) {
          alert("Error while reading file " + file.name + ": " + loadEvent.target.error);
          return;
        }
        console.log(loadEvent.target.result.length); // Your text is in loadEvent.target.result
      };
      loader.readAsText(file);
    })(changeEvent.target.files[i]);
  }
});

Worth noting:

• You must use one FileReader for each (concurrent) file read. Otherwise you see an exception like The object is already busy reading.
• The loadEvent callbacks will be called in an arbitrary order, likely dependent on the size of the upload.
• The loadEvent closure will see the i value which ended the loop.
• FileReader results are NOT arrays; they have no forEach.

This jsfiddle demo preserves upload order by laying out divs in the change handler.

Here is a quick and dirty example based on a form named "myform" that contains a file input named "myfile" :

document.forms['myform'].elements['myfile'].onchange = function(evt) {
    if(!window.FileReader) return; // Browser is not compatible

    var reader = new FileReader();

    reader.onload = function(evt) {
        if(evt.target.readyState != 2) return;
        if(evt.target.error) {
            alert('Error while reading file');
            return;
        }

        filecontent = evt.target.result;

        document.forms['myform'].elements['text'].value = evt.target.result;
    };

    reader.readAsText(evt.target.files[0]);
};

Here's the associated HTML form:

<form id="myform">
  <p>
    <input id="myfile" name="files[]" multiple="" type="file" />
    <textarea id="text" rows="20" cols="40">nothing loaded</textarea>
  </p>
</form>

and a jsfiddle to demo it.