Type issue in oneOf schemas

[sceccotti89]

In a oneOf schema, the most nested property is not properly typed by TS.

Example:

const catSchema = {
  type: 'object',
  properties: {
    cat_property: {
      type: 'object',
      oneOf: [
        {
          properties: {
            name: { type: 'string' },
          },
          required: ['name'],
        },
        {
          properties: {
            color: { type: 'string', enum: ['black', 'brown', 'white'] },
          },
        },
      ],
    },
  },
} as const;

type Cat = FromSchema<typeof catSchema>;
const cat: Cat = { cat_property: { name: 'My name' } };
cat.cat_property.name; // This is considered unknown instead of string.

If I hover over the Cat type, everything looks fine:

type Cat = {
    [x: string]: unknown;
    cat_property: {
        [x: string]: unknown;
        name: string;
    } | {
        [x: string]: unknown;
        color?: "black" | "brown" | "white" | undefined;
    };
}

But when I try to access the name attribute, TS says it's unknown.

Is it a bug of the library or it's due to Typescript?

[giraffesyo]

Looks like you need to set additionalProperties: false,, along with the oneOf.

[sceccotti89]

@giraffesyo

Thanks for the response but it's still not working. This is the updated schema:

const catSchema = {
  type: 'object',
  additionalProperties: false,
  oneOf: [
    {
      additionalProperties: false,
      properties: {
        name: { type: 'string' },
      },
      required: ['name'],
    },
    {
      additionalProperties: false,
      properties: {
        color: { type: 'string', enum: ['black', 'brown', 'white'] },
      },
    },
  ],
}

I tried all the possible combinations (with and without additionalProperties) but none of them worked. Were you able to test it yourself and make it works?

I forgot to share details about my environment.

• Typescript v4.9.5
• node v16.17.1

[ThomasAribart]

Hi @sceccotti89 !

Several elements here:

• The second schema you provided will not really work as the additionalProperties at the root level will apply to an "invisible" properties: {} keyword, and thus deny name and colorproperty. The only valid data is { color?: undefined }. You probably want to remove it.
• In the first schema, the problem you encounter is due to Typescript. Technically, the cat_property is a union of two cases, A (name) or B (color). In your context, event if your cat const has a name property, it is typed as Cat and TS will not take it for granted. Thus, when you try to access the name property, TS still considers that it can be string in case A or unknown in case B. string is a particular case of unknown so unknown wins.

You can try not to assign the Cat type to your cat const but simply make sure it satisfies it:

// Will type-check cat against Cat type, but keep the "naturally inferred" cat type
const cat = { cat_property: { name: 'My name' } } satisfies Cat

Does that answer your issue ?