Nowadays you should use modules. Put your code into the default function of a module and import the function into a script element.
client.js
:
export default function ()
{
alert ("test");
}
index.html
:
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>test</title>
</head>
<body>
<script type="module">
import main from './client.js';
main ();
</script>
</body>
</html>