Union Error Message

When one passes in data that satisfies no conditions for a union the error message is rather unhelpful. Consider the following, where we neglect to provide a target field on the either object({}) provided to union([]).

const { assert, object, number, string } = window.Superstruct
const { union } = window.Superstruct;

const Validation = union([
  object({ documentID: string() }),
  object({ formID: string() }),
]);

const data = {
  target: 'DOCUMENT',
  documentID: 'identifier',
}

The resulting error message looks as follows, to be contrasted with the error message against a raw object({}):

Expected the value to satisfy a union of `object | object`, but received: [object Object]

At path: target -- Expected a value of type `never`, but received: `\"DOCUMENT\"`

[ianstormtaylor]

@hgwood thanks for the helpful write up! I like that idea with the extra context of the first failure—I believe it should use failure.path instead of failure.key in case it’s deeply nested. If you wanted to pull request that would be great!

For Example 2 it would be interesting to know what TypeScript does without the interfaces defined, so that it can’t just say A | B since Superstruct doesn’t know those nicer names usually.

[hgwood]

Thanks. I might try this.