Object

Use QueryLogic.UNDEFINABLE to represent that a set exists, but it can not be represented by the available set types. For example, consider performing a difference of people in Chicago compared to people named Justin who are 35:

import {QueryLogic} from "can";

const inChicago = new QueryLogic.KeysAnd( {location: "Chicago"} );
const isJustinAnd35 = new QueryLogic.KeysAnd( {name: "Justin", age: 35} );

const undefinable = QueryLogic.difference(inChicago, isJustinAnd35);
console.log( undefinable ); //-> QueryLogic.UNDEFINABLE

If QueryLogic supported a Not and an expanded ValueAnd type, the difference could be described as:

import {QueryLogic} from "can";

const inChicago = new QueryLogic.KeysAnd( {location: "Chicago"} );
const isJustinAnd35 = new QueryLogic.KeysAnd( {name: "Justin", age: 35} );

const undefinable = new QueryLogic.ValueAnd([
  inChicago,
  new QueryLogic.Not(isJustinAnd35)
]);
console.log( undefinable );

Currently, as QueryLogic does not fully support Not and ValueAnd used this way, it returns QueryLogic.UNDEFINABLE. This indicates a difference exists (the existence of a difference is important for determining superset and subset), but we aren't able to express it.