Angular custom function, if(value == value) is treated as value LIKE value

I'm pretty new to angular so please be gentle.

the problem:

i have a custom filter for a dropdown using ng-repeat on the options

<select ng-model="SelectPart">
    <option ng-repeat="part in parts| filter:customFunction" value="{{ part.id }}">{{ part.name }}</option>
</select>

here is the custom function:

$scope.customFunction = function(item){
    if (item.parent_part_id == 2) {
        return true;
    }else if(item.id == 2){
        return true;
    };
    return false;
};

now here is my question:

Why does the function return true when the part.id is 12?

how come that when i say == in an if, it acts more as if it were a LIKE statement in mysql?

what can i do to fix this?