To set the width of the top border in JavaScript, use the borderTopWidth property. Set the width of the top border using this property.
Example
You can try to run the following code to learn how to set the width of the top border with JavaScript −
<!DOCTYPE html>
<html>
<head>
<style>
#box {
border: thick solid blue;
width: 300px;
height: 300px;
}
</style>
</head>
<body>
<div id="box">Demo Text</div>
<br><br>
<button type="button" onclick="display()">Change top border width</button>
<script>
function display() {
document.getElementById("box").style.borderTopWidth = "10px";
}
</script>
</body>
</html>