Use the fontWeight property in JavaScript to set the font boldness.
Example
You can try to run the following code to set the boldness of the font with JavaScript −
<!DOCTYPE html>
<html>
<body>
<h1>Heading 1</h1>
<p id="myID">
This is Demo Text.
</p>
<button type="button" onclick="display()">Set Font Family and Font Weight</button>
<script>
function display() {
document.getElementById("myID").style.fontFamily = "verdana,sans-serif";
document.getElementById("myID").style.fontWeight = "800";
}
</script>
</body>
</html>