сценарий в другом файле

js/script.js

window.onload=function() {
document.getElementById('d1').innerHTML='<h1>Hello World!</h1>';
}
function funMake() {
document.write('<h1>Привет Мир!</h1>');
}

window.onload=function() {
 document.getElementById('d1').innerHTML='Hello World!';
}

function funMake() {
 document.write('Привет Мир!');
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<div id="d1"></div>
<script>
funMake();
</script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<div id="d1"></div>
<script>
funMake();
</script>
</body>
</html>