Javascript fumktioniert in console aber nicht innerhallb einer php Datei

document.querySelector("#groesse").onchange = function (e) {  
// some things
alert("Changed to " + this.value);  
}

<!-- start Simple Custom CSS and JS -->
<script type="text/javascript"> 
document.querySelector("#groesse").onchange = function (e) { 
// some things
alert("Changed to " + this.value); 
}
</script>
<!-- end Simple Custom CSS and JS -->