Tryit Editor v2.9

Edit This Code:
<!DOCTYPE html>
<html>
<body>

<div id="myDiv">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Set left border</button>

<script>
function myFunction() {
    document.getElementById("myDiv").style.borderLeft = "thick solid #0000FF";
}
</script>

</body>
</html>