Class Problem

I have a problem with classes in javascript. When I define my class and
make an instance more than one time, it does not contian their own
variables:

var Counter = (function() {
var counter = 0;

function Constructor() {
this.fncInit();
}

Constructor.prototype.fncInit = function() {
counter++;
alert(counter);
}
})();

var counter1 = new Counter();
var counter2 = new Counter();
var counter3 = new Counter();

The counter increases by one each time, but it should always be 1.