simple file upload to base64 string

index.html

<div>
    <label for="filePicker">Choose or drag a file:</label><br>
    <input type="file" id="filePicker">
</div>
<br>
<div>
    <h1>Base64</h1>
    <textarea id="base64str" cols="50" rows="15"></textarea>
</div>
<script>
var fileHandler = function(evt) {
    var files = evt.target.files;
    var file = files[0];

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

        reader.onload = function(readerEvt) {
            var binaryString = readerEvt.target.result;
            document.getElementById("base64str").value = btoa(binaryString);
        };

        reader.readAsBinaryString(file);
    }
};

document.getElementById('filePicker').addEventListener('change', fileHandler, false);
</script>