Use the document.title property to set the title of the document in JavaScript.
Example
You can try to run the following code to implement document.title property in JavaScript.
<!DOCTYPE html>
<html>
<head>
<title>Demo Title</title>
</head>
<body>
<script>
var x = document.title;
document.write("Our Title: "+x);
</script>
</body>
</html>