aoc-js-boiler.html

<input type="file" id="inputfile">
<br>
<pre id="output"></pre>

<script type="text/javascript">
    document.getElementById('inputfile')
            .addEventListener('change', function() {

        var selected_file = new FileReader()

        selected_file.onload = function(){
            // let data = fr.result.trim().split('\n').map(x => parseInt(x))
            document.getElementById('output').textContent = selected_file.result
        }

        selected_file.readAsText(this.files[0]);
    })
</script>