how to use google.script.run as if it was a function

In a Google Apps Script, I have the following script:

function doGet() {
  return HtmlService.createHtmlOutputFromFile('mypage');
}

function writeSomething() {
  return "<h1>hi people</h1>";
}

and the following html file:

<html>
  <a id="caller" href="#">update</a>
  <br>
  <div id="div">waiting...</div>
<script>
function go() {
  var a=google.script.run.writeSomething();
  document.getElementById("div").innerHTML=a;
}
document.getElementById('caller').onclick = go;
</script>
</html>

When I click on the "update" link, the div content changes from "waiting..." to "undefined". I suppose that google.script.run cannot be called as a function. So, how can I solve this trouble? (obviously, this is a toy example; I need a way to update the div content from the script and not from the html file)

how to use google.script.run as if it was a function

1 Answers