Function()

js/script.js

var fun = Function("document.write('Hello World!<br />');");

var fun = Function("document.write('Hello World!
');");

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<script>
fun();
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<script>
fun();
</script>
</body>
</html>