can-query-logic/doc/static/undefinable.md

query data

@property {Object} can-query-logic.UNDEFINABLE UNDEFINABLE
@parent can-query-logic.static-types

Represents a set that contains values, but can not be defined and is not the [can-query-logic.UNIVERSAL] set.

@type {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:

  ```js
  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
  ```
  @codepen

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

  ```js
  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.

  <!-- Use [can-query-logic.UNKNOWABLE] to represent -->