Some C++ programmers expect JavaScript to support passing parameters by reference. When we pass a parameter by reference, we pass the address of the parameter instead of its value. Once inside the function, we can change the stored value in this address. If JavaScript would have supported passing by reference, the following script would have resulted in an alert box showing the value of 6 (3 is passed to the function change() where it is doubled):

<SCRIPT LANGUAGE="JavaScript">
var a = 3;
change(a);
alert(a);

function change(value) {
value = value * 2;
}

</SCRIPT>
But JavaScript does not support passing by reference. It passes its parameters by value only. A function does not have any way to know the address of its parameters, and hence cannot change the stored parameter value. The script above generates an alert box with the value of 3, as the variable a is not being changed by the function change().