POST a js::array in JSON, got "undefined:undefined"

POST a js::array in JSON, got "undefined:undefined"

I have a javascript array in the frontend, and I want to post it to my
Tomcat server.
I'm using this code but it's not working as
console.log("preview list. postUsers()");
console.log(Ext.getCmp("preview-container").getUsers());
$.ajax({
url: "url",
type: "POST",
data: Ext.getCmp("preview-container").getUsers(),
dataType: "json",
success: function(rst){
switch (rst){
case true:
alert("success");
break;
case false:
alert("failed");
break;
}
},
});
The object is like this:
preview list. postUsers()
["18241", "44598", "46558"]
But the POST result is like this:
undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&undefined=undefined&
Which is parsed like this:
undefined: undefined ...
Am I missing anything important?