Q1590 Does JavaScript do 'call by reference' or 'call by value'?

<script language="JavaScript"><!--
function byValue(a_variable) {
  a_variable *= a_variable;
}

function byReference(an_object) {
  an_object.a_property *= an_object.a_property;
}

var x = 12;
byValue(x);
document.write(x + '<br>'); // displays 12

var y = new Object();
y.a_property = 12;
byReference(y);
document.write(y.a_property + '<br>'); // displays 144
//--></script>