<input type="file" id="files" />

<table id="myTable">
<tbody id="myTBody">
</tbody>
</table>


<script>
function doit(evt) {

    let file = evt.target.files[0];
    let reader = new FileReader();
    reader.readAsText(file);
    reader.onload = (evt) => {

        let lines = evt.target.result.split('\n');
        for (var i = 0; i < lines.length; i++) {
            if (lines[i] === "")
                continue
            let myStyle = "";
            if (lines[i].indexOf("let") != -1) {
                myStyle = "background:red";
            }
            document.getElementById('myTBody').innerHTML += '<tr > <td style=' + myStyle + '>' + lines[i] +  '</td></tr>';
        }
    }
}

document.getElementById('files').addEventListener('change', doit, false);
</script>