SetInterval Inside A Class

how can i set an interval inside a class

exmpl.

var Class = (function() {
var interval = undefined;

function initInterval(instance) {
interval = setInterval("doSomething", 1000, instance);
}

function doSomething(instance) {
alert(instance.message);
}

function Constructor() {
this.message = "hello world";
}

Constructor.prototype.init = function() {
initInterval(this);
}

return Constructor;
})();

is not working correct. doSomething is not known. i think, setInterval
is not inside the class, so doSomething is private and unknown for the
interval.