Fix #56: Better error messages for nullable types

[oleg-slapdash]

This fixes https://github.com/woutervh-/typescript-is/issues/56 by providing a better error message when a type is nullable. It only changes behaviour when a union contains 2 members (e.g. won't change behaviour for a | b | c) and when one of the union members is null null | a.

reateAssertType<{
  data: {
    node: { a: string; closed: string; b: string } | null;
  };
}>()({ data: { node: { a: "a", closed: false, b: "b" } } });

// Before:
// TypeGuardError: validation failed at $.data.node: there are no valid alternatives
// After: 
// TypeGuardError: validation failed at $.data.node.closed: expected a string, found: false

[DetachHead]

does this only work on null? it should probably also do this for undefined / optional fields aas well

[oleg-slapdash]

@DetachHead It works only with null, it does not work with Array<Type | undefined>, but it works with interface Type {optional?: OtherType | null}

[dko-slapdash]

@woutervh- any plans on merging this PR? We’ve been using this patch for half a year so far (we keep a private fork of this module just in same of the single patch), it works well and is very useful in practice.

[dko-slapdash]

@woutervh- - any plans to merge? We have to support our own fork at @slapdash/typescript-is for just this simple change, would be very convenient if it's merged directly into the official module.

[woutervh-]

Hi @dko-slapdash

Apologies for being late. Thanks for the reminder, I will merge and release this now :)