[Como] posso conseguir o seguinte?

var object = new Object();
object.field1 = "test1";
object.fiedl2 = "test2";

function showSpecificField(fieldName){
    //get the field Name passed in to function from object
    //something like: return object.fieldName ???
}

alert(showSpecificField("field2"));// should alert the text "test2"

alert(showSpecificField("field1"));// should alert the text "test1"