The code that you have would work, but is executed from the global context, which means that this refers to the global object.
<script type="text/javascript">
var foo = function(param) {
param.innerHTML = "Not a button";
};
</script>
<button onclick="foo(this)" id="bar">Button</button>
You can also use the non-inline alternative, which attached to and executed from the specific element context which allows you to access the element from this.
<script type="text/javascript">
document.getElementById('bar').onclick = function() {
this.innerHTML = "Not a button";
};
</script>
<button id="bar">Button</button>