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