Wrong enum name when using union with null

[bdeneux]

I've an issue when trying to convert a JSON schema file to Typescript types. The JSON schema comes from a Rust models that has been converted to JSON Schema.

Given this schema : (I've simplified the schema for this exemple)

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "ConstructResponse",
  "description": "Represents the response of a [QueryMsg::Construct] query.",
  "type": "object",
  "required": [
    "format"
  ],
  "properties": {
    "format": {
      "description": "The format of the data.",
      "anyOf": [
        {
          "$ref": "#/definitions/DataFormat"
        },
        {
           "type": "null"
        }
      ]
    }
  },
  "additionalProperties": false,
  "definitions": {
    "DataFormat": {
      "title": "DataFormat",
      "description": [...],
      "oneOf": [
        {
          "title": "Turtle",
          "description": [...]
          "type": "string",
          "enum": [
            "turtle"
          ]
        },
        {
          "title": "RDF XML",
          "description": [...],
          "type": "string",
          "enum": [
            "rdf_xml"
          ]
        },
        {
          "title": "N-Triples",
          [...]
        },
        {
          "title": "N-Quads",
          [...]
        }
      ]
    }
  }
}

And running QuickType to convert this json schema in Typescript types,

quicktype -s schema schema.json -o test.ts --prefer-type

The conversion use the first enum case as the name of the enum, here the enum will be named Turtle instead of DataFormat.

/**
 * Represents the response of a [QueryMsg::Construct] query.
 */
export type Test = {
    format: Turtle | null;
}

export enum Turtle {
    NQuads = "n_quads",
    NTriples = "n_triples",
    RDFXML = "rdf_xml",
    Turtle = "turtle",
}

Expected result

/**
 * Represents the response of a [QueryMsg::Construct] query.
 */
export type Test = {
    format: DataFormat | null;
}

export enum DataFormat {
    NQuads = "n_quads",
    NTriples = "n_triples",
    RDFXML = "rdf_xml",
    Turtle = "turtle",
}

When I remove { "type": "null" } from the anyOf property of DataFormat, the enum is named correctly.

JSON without the `null` union

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "ConstructResponse",
  "description": "Represents the response of a [QueryMsg::Construct] query.",
  "type": "object",
  "required": [
    "format"
  ],
  "properties": {
    "format": {
      "description": "The format of the data.",
      "anyOf": [
        {
          "$ref": "#/definitions/DataFormat"
        }
      ]
    }
  },
  "additionalProperties": false,
  "definitions": {
    "DataFormat": {
      "title": "DataFormat",
      "description": [...],
      "oneOf": [
        {
          "title": "Turtle",
          "description": [...]
          "type": "string",
          "enum": [
            "turtle"
          ]
        },
        {
          "title": "RDF XML",
          "description": [...],
          "type": "string",
          "enum": [
            "rdf_xml"
          ]
        },
        {
          "title": "N-Triples",
          [...]
        },
        {
          "title": "N-Quads",
          [...]
        }
      ]
    }
  }
}

Give a good enum name but is not what I want because DataFormat is optional :

/**
 * Represents the response of a [QueryMsg::Construct] query.
 */
export type Test = {
    format: DataFormat;
}

export enum DataFormat {
    NQuads = "n_quads",
    NTriples = "n_triples",
    RDFXML = "rdf_xml",
    Turtle = "turtle",
}

This null type union has been added because format property can be null since this attribute is optional in rust.

I hope you can help me, I'm wondering if it's an issue in this library for this particular case or if it's the JSON Schema that has been malformed by the rust library.

[inferrinizzard]

Hi @bdeneux, there looks to be 2 issues at play here:

1. There does indeed seem to be a naming issue here with the Enum, which I will take a look at
2. For an optional property on the format object, an easier representation would just be to omit the key from the required array - although the anyOf with null is also correct (and might make more sense coming from rust)

[schani]

Triage sweep note (systematic issue-backlog triage, assisted by Claude): confirmed reproducible on current master (v23.3.0, 68ecb284), TypeScript target.

$ quicktype -s schema schema.json --lang ts --prefer-types -o out.ts

produces

export enum Turtle {
    NQuads = "n_quads",
    NTriples = "n_triples",
    RDFXML = "rdf_xml",
    Turtle = "turtle",
}
...
format: Turtle | null;

instead of the expected DataFormat enum name. Also reproduces in Go, confirming the defect is in quicktype-core, not a TS-renderer issue.

Root cause: the "DataFormat" title is attached to the oneOf union of four single-value string enums. When the schema flattens this union with null (via anyOf), FlattenUnions.ts calls unifyTypes with emptyTypeAttributes, and the four variant enums get fused into one enum that only carries the variant titles (Turtle, RDF XML, ...); the DataFormat title itself is left attached to the outer, now-discarded inline union rather than propagated to the merged enum. The enum's naming logic then falls back to the first variant's title.

Assessment

• Severity: low. Output still compiles and round-trips correctly — this is a naming/readability defect only.
• Difficulty: hard. Fixing it requires correct title-attribute propagation through unifyTypes/FlattenUnions, which interacts with the existing name-distance/alternative-name heuristics in TypeNames.ts. A naive fix risks regressing enum/type naming across many golden fixtures.

Repro input (schema.json):

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "ConstructResponse",
  "description": "Represents the response of a [QueryMsg::Construct] query.",
  "type": "object",
  "required": ["format"],
  "properties": {
    "format": {
      "description": "The format of the data.",
      "anyOf": [
        { "$ref": "#/definitions/DataFormat" },
        { "type": "null" }
      ]
    }
  },
  "additionalProperties": false,
  "definitions": {
    "DataFormat": {
      "title": "DataFormat",
      "description": "The format of the data.",
      "oneOf": [
        { "title": "Turtle", "description": "Turtle format", "type": "string", "enum": ["turtle"] },
        { "title": "RDF XML", "description": "RDF XML format", "type": "string", "enum": ["rdf_xml"] },
        { "title": "N-Triples", "description": "N-Triples format", "type": "string", "enum": ["n_triples"] },
        { "title": "N-Quads", "description": "N-Quads format", "type": "string", "enum": ["n_quads"] }
      ]
    }
  }
}