How to Filter dropdown value via code

I have a code to fetch value from Item Type Project. Below is the code:

In this code I fetch all the project name in a dropdown style in similar_project property but i have a query regarding if i want to filter by department wise project name then How will do it via this code or what modification required.

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var div_id = getFieldByName("similar_project").id;

var control_id = div_id.substring(0, div_id.indexOf("span"));

var select = document.getElementById(control_id);

//...Replace this with a call to a external datasource...

//get Customer

var returnItm = document.thisItem.newItem("Project", "get");

returnItm.setAttribute("select","name");

returnItm = returnItm.apply();

//populate the select list with options

for (var i=0;i < returnItm.getItemCount(); i++)

{

                var project = returnItm.getItemByIndex(i);

                select.options[i]=

                new Option(project.getProperty("name"),

                project.getProperty("id"));

}

Thanks in advance

Abhishek Srivastava