How can I append values to an object, if I have this:

Code JavaScript:
var fields_array = new Object();
 
function somefunction(){
        fields_array.fieldset = groupname;
	fields_array.fieldsetname = groupnamelegend;
	fields_array.field = newfield;
}

I cant figure out how to add multiple values so I have something like
Code JavaScript:
//index 1 of object
fields_array.fieldset = 'somevalue';
fields_array.fieldsetname = 'somevalue';
fields_array.field = 'somevalue';
//index 2 of object
fields_array.fieldset = 'somevalue';
fields_array.fieldsetname = 'somevalue';
fields_array.field = 'somevalue';
//etc