To set the style of the left border in JavaScript, use the borderLeftStyle property. Set the style under this property that you want for the border i.e. solid, dashed, etc.
Example
You can try to run the following code to learn how to style the left border −
<!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 left border style</button>
<script>
function display() {
document.getElementById("box").style.borderLeftStyle = "dashed";
}
</script>
</body>
</html>