JS：在屏幕上显示HelloWorld;
<!DOCTYPE html>
<html>
<head>
<script>
function myFunction()
{
document.getElementById("demo").innerHTML="My HelloWorld JavaScript Function";
}
</script>
</head>
<body>
<h1>My Web Page</h1>
<p id="demo">A Paragraph.</p>
<button type="button" onclick="myFunction()">点这里</button>
</body>
</html>