onload

js/script.js

function funShow() {
document.body.childNodes[5].innerHTML = '<h1>Hello World!</h1>';
}
window.onload = function() {
document.body.childNodes[1].onclick = funShow;
}

function funShow() {
document.body.childNodes[5].innerHTML = '<h1>Hello World!</h1>';
}
window.onload = function() {
document.body.childNodes[1].onclick = funShow;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input type="button" value="OK" />
<br />
<div></div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<script src="js/script.js"></script>
</head>
<body>
<input type="button" value="OK" />
<br />
<div></div>
</body>
</html>